SPE 153580

Milestone in Production Using Proactive Azimuthal Deep-Resistivity Sensor Combined with Advanced Geosteering Techniques: Tarapoa Block, Ecuador Alex Guevara, Andes Petroleum; Juan Sandoval, Manuel Guerrero, and Carlos A. Manrique, Halliburton

Copyright 2012, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Latin American and Caribbean Petroleum Engineering Conference held in Mexico City, Mexico, 16–18 April 2012. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract A recent challenge of drilling a horizontal well in a M1 Sandstone reservoir presented high seismic uncertainty and limited lateral extent; this well is located in the Napo formation of the Oriente basin in Ecuador. The geology in this basin is complicated; most of the reservoirs are formed from a small, stacked-channel sandstone sequence. As a result, well-to-well reservoirs are difficult to correlate because of their limited lateral extent. Horizontal wells placed in this kind of environment generally require adjustments in the planned directional well trajectory and modification of the navigation TVD when required.

The primary goal of this project was to maintain the well in the sweet spot of the reservoir to improve productivity. As a final delivery, the structural map of the top of the M1 Sandstone enabled the customer to adjust the seismic information in the zone of influence of the well. A major drilling company assumed the challenge by using a rotary steerable system, proactive logging-while-drilling azimuthal resistivity sensors, and 3D geosteering techniques to place the well in the sweet spot of the reservoir and to ensure the permanence in the sandstone reservoir.

The azimuthal deep resistivity sensor can provide a broad quantity of curves with various depths of investigation (DOIs). Having this information as an entry, the geoscientists applied the three logical geosteering phases of model, measure, and optimize. At the modeling stage, the geosteering team selects the appropriate proactive set of variables to transmit in real time, including compensated resistivities at various ranges of investigation, images, and geosignals according to the geology in the area, reservoir thickness, and existing resistivity contrast. The measuring stage begins by obtaining the selected variables in real time with average resistivities that enable the calculations of the distance-to-bed boundary (DTB) using a forward-modeling technique, while real-time images are compared against modeled information for stratigraphycal positioning. During landing, the drilling and geology departments agreed that the reservoir top was 35 ft (10.7 m) shallower than expected. At this point, the directional drilling plan needed to be changed, beginning the optimization stage even before the horizontal section began.

The appropriate combination of reactive and proactive logging-while-drilling sensors enabled the well to be placed parallel to the top of the reservoir, maintaining an optimal distance of 1 to 3 ft, with 100% reservoir exposure in the pay zone and no exits.

The main objectives of geosteering were achieved. The well produced 6,800 BOPD after an initial estimate of 800 BOPD. The top of the reservoir was mapped, thereby improving knowledge of this zone for future study. Introduction Geosteering, as a wellbore placement methodology, is currently an important topic. The use of the geosteering technique has increased exponentially during recent years, and is imperative during the evaluation of a new project. Geosteering is used in most of the horizontal/near-horizontal, high-angle wells in which tolerances are generally small. These types of wells require high-end LWD tools and a proactive forward modeling software that integrates the petrophysical responses obtained from those sensors with the available geological information. This integration enables the specialist to accurately place the well in a highly deviated segment of the sweet spot of the formation reservoir. The geosteering concept can be summarized as the act of adjusting the borehole position on-the-fly to reach one or more geological targets; these adjustments are based on the petrophysical information gathered while drilling. (Lesso et al. 1996)

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As a proactive method, geosteering uses a forward modeling software and a combination of LWD sensors responses (in which the azimuthal deep resistivity is the most important sensor along the horizontal/highly deviated section), in addition to drilling systems and a group of highly trained specialists within a real-time environment. Geosteering provides the following primary benefits:

Mitigate risks before and during drilling operations, improving decision speed and reducing uncertainties Maximize the production rate, extending asset longevity Decrease development costs

This paper describes a geosteering method based on azimuthal resistivity information. It demonstrates how this proactive

method helped to steer a well to remain within the sweet spot of the reservoir, yielding a successful well with maximum reservoir contact. The additional information from deep azimuthal wave resistivity and the deep images are key factors in this application, enabling the well to be maintained within the boundaries of the reservoir over long intervals. Geosteering in Complex Depositional Environments That many formations are laterally discontinuous provides complex geological scenarios that complicate the precise placement of wellbores. In these scenarios, the depth of investigation, rather than distance from the bit, is not only vital, but the most important factor. Generally, azimuthal deep resistivity measurements are used to locate bed boundaries and fluid contacts. In uniform porosity sands, the resistivity log detects the possibility of accidentally drilling into the water leg before it actually occurs, which provides sufficient time to react and correct the well trajectory. The multiple resistivity readings from the sensor, each having a different depth of investigation, senses when the wellbore is approaching a bed or has a distinctly different resistivity from the bed currently being drilled (e.g., going from a high-resistivity sand to a rather low-resistivity shale or approaching an oil-water contact). As a result of the considerable depth of investigation of this sensor at the formation resistivities (Rt), it can sense a boundary before the bit actually penetrates the new formation or crosses the fluid contact. This method is more effective at sensing bed boundaries ahead of time than “at-bit” resistivity readings, resulting from the combination of depth of investigation and the low-angle approach to the bed boundary. The petrophysical data obtained from deep azimuthal wave resistivity and gamma ray sensors with at-bit directional information obtained have been used in conjunction with real-time interactive forward modeling/geosteering software to optimize formation evaluation, geosteering, and geocorrelation in the Tarapoa area (Bejarano et al. 2010). Geographical and Geological Framework The Alice field is located in the western area of the Tarapoa block, in the hanging wall of the normal fault that limits the Fanny and Dorine fields (Fig. 1) of the Oriente basin in Ecuador. Alice field was discovered in July 2002; it is M1 Sandstone, and the only reservoir in the field.

Fig. 1—Geographical location of Alice field.

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The Upper Cretaceous Napo Formation, which contains the M1 Sandstone, consists of a series of marine shales, limestones, and marginal marine sandstones that are interspersed in cyclic sequences. Fig. 2 shows a stratigraphic column of the Oriente basin.

Fig. 2—Oriente Basin stratigraphical column.

M1 Sandstone As previously mentioned, the M1 Sandstone rock is the only reservoir in this structure; this sandstone represents the earliest depositional cycle of the Napo formation, which overlaps in an erosive contact offshore type marine shales reaching thicknesses of more than 50 ft in the area.

M1 Sandstone sediments were deposited in a transitional estuarine depositional environment, influenced by tides during the course of a prolonged drop and subsequent rise in the mean sea level. This sandstone represents a complex sequence dominated by tidal channels gradually converted in abandoned filled tidal channels and tidal flats sequences. The Dorine-Fanny-Alice area can be subdivided into an upper and a lower sequence. The lower section of the M1 (M1Inferior) represents the sequence dominated by tidal channels; the upper section (M1 Superior) represents the abandoned filled channel and tidal flat sequences phase. (Muñoz 2007)

The entrapment mechanism of the M1 Sandstone in the Alice field has been interpreted as a combination of up-dip facies to impermeable facies change, combined with a structural closure, as shown in Fig. 3.

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Fig. 3—Seismic horizons and geological markers for Alice field.

Geosteering Techniques A combined proactive geosteering technique, based on the use of the azimuthal resistivity tool as the main tool for navigation, was used with gamma ray real-time petrophysical information and a 3D interactive correlation software. This combination enabled the geology of the area to be modeled and the theoretical response of the tool, based on offset wells, measured the real-time petrophysical properties and optimized the planned trajectory on-the-fly to obtain the best wellbore position into the M1 Sandstone. Definition. Geosteering methods are traditionally classified as “reactive” and “proactive” (Fig. 4). Geosteering is reactive when the steering decision is driven by one or more events that have already occurred, such as the crossing of a reservoir boundary or a thin-shale bed. Geosteering is proactive when the steering decision is driven by the early information of an impending event. Because of their significant depths of investigation, traditional wave resistivity tools have paved the way for proactive geosteering. Petrocedeño used the azimuthal deep-resistivity sensor as the main driver, which provides the major benefit of this technique (Al-Mutari et al. 2009).

Fig. 4—Difference between proactive and reactive geosteering: source of comparison resistivity vs. gamma ray sensors.

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Alice 16H Case Study

This section describes the well objectives and the well planning stage of the Alice 16H well. Well Objectives. The well objectives of the Alice 16H well include the following:

Position the well in the best possible zone of the M1 Sandstone reservoir (M1 SST), an area that presents high seismic and stratigraphic uncertainty.

Map the top of the M1 Sandstone through the calculation of the distance-to-bed (DTB) to adjust seismic studies in the zone.

Navigate with a horizontal section at the uppermost part of the reservoir with a maximum length of 235 m (770 ft) into the M1 SST.

The horizontal section navigated five (5) ft below the M1 top, maintaining 90 degrees of inclination, to find the high

structural position in Alice 1. The expected oil net pay thickness estimated for M1 sand at the Alice 16H well is approximately 47 ft thick from top to base, assuming that the OWC is be similar to that at the Alice 12 well (-7,127 ft SSTVD) (Fig. 5).

Fig. 5—Alice field: Top M1 SST structural map. Well Planning Stage. To achieve the objectives of this project, a multidisciplinary team was formed to define the level and type of uncertainty in a zone with very little geological and seismic information. Once defined the limiting factors main challenges were foresaw and main entry data was provided for optimal well bore positioning:

Offset data: Alice 12 and Alice 01 as correlation wells using Gamma Ray and Resistivity wireline petrophysical information to be projected/modeled having as a reference the target formation and projected well plan.

Geology: The use of the available interpreted geological surface (M1 Sandstone Top)

Due to a lack of detailed seismic information, a main objective was to map the top of the M1 SST by calculating the distance-to-bed boundary (DTB) with a 3D interactive correlation software that uses geosignals and certain azimuthal resistivity information obtained from the azimuthal deep resistivity sensor.

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Fig. 6—Software-generated image of surfaces and trajectory of the ALICE 16H well and ALICE 12 offset.

The target reservoir is defined as the M1 Sandstone (M1 SST). According to the stratigraphy in the area, an initial well

plan was defined that ranged from 4 to 5 ft TVD at the top of the reservoir to a navigation depth of 7,879 ft TVD. After better characteristics in this zone of the reservoir were observed, a new well plan was attempted, maintaining the well trajectory parallel to the top, minimizing effects of the well geometry, and avoiding an undesired formation at the base of the reservoir.

The log presented in Fig. 7 shows an important lithological marker at the entrance of M1 Principal by the presence of a shale neck followed by the reservoir; this neck is the seal that extends toward the east. The M1 Principal Sandstone body is marked by low gamma ray values and a drastic rise in resistivity that exhibits the necessary contrast that enables the azimuthal deep resistivity tool to fulfill its function. Within M1 Principal and with sand thicknesses ranging between 30 and 35 ft, a navigation zone of approximately 10 ft TVD with optimum reservoir characteristics was defined.

Fig. 7—Gamma ray and resistivities: petrophysical parameters to identify top of the M1 main sandstone, Alice 16H well.

Within the area of geosteering, the responses provided by pre-defined azimuthal resistivity readings at certain depth of investigation (DOIs) and geosignals pulsed by the azimuthal deep resistivity sensor enabled very reliable positioning within the producing interval. The 3D interactive correlation software provided the means of integrating geological and petrophysical information for optimized wellbore positioning. The resistivity contrast at the top of the reservoir enabled the mapping of the reservoir top for future adjustments in the local seismic campaign conducted in the area. Proactively Geosteering Using Azimuthal Deep Resistivity Data Four main elements obtained as products or subproducts of the LWD azimuthal deep resistivity sensor are used to proactively position a given well in the best zone of the reservoir. These elements include the following:

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1. Resistivity Top and Bottom Deep Readings. A deep compensated resistivity reading was transmitted in real time and used in combination with other parameters to geosteer within the evaluated units (M1 Sandstone Napo formation). A 48-in., 500-kHz phase resistivity reading enabled a full resistivity image that could be decomposed in bins or sectors. A point-to-point evaluation of the top and bottom readings (sectors) helped the interpreter to obtain a sectorized vision of what was happening above and below the wellbore in terms of resistivity and to build or drop the inclination angle when required (Bittar et al. 2007). (Fig. 8).

Fig. 8—Proactive geosteering with azimuthal-wave resistivity. In a reservoir, the up-resistivity curve and the down-resistivity curve exhibit a predictable separation. The signature clearly distinguishes between the various cases where the well is approaching the conductive interval from above, from below, or from the side.

2. Resistivity Images and Bright Spots. As a quick overview, one qualitative quick-look interpretation of

stratigraphical position and wellbore images consists of focusing on sinusoidal patterns and classifying them as either “smiling” or “frowning” patterns. Frowning patterns indicate that the well is being drilled down-dip or down-structure, with respect to the geological event; smiling patterns indicate that the well is being drilled up-dip or up-structure (Chemali et al. 2009) (Fig. 9).

Fig. 9—Azimuthal image reveals frowning and smiling patterns as indications of the stratigraphical position.

This concept, combined with the bright-spot phenomenon in an azimuthal deep resistivity image, provides the interpreter

with a better understanding of the position within the reservoir. This bright-spot phenomenon occurs in an induction-type azimuthal resistivity image and is an “image extension” of the horn effect. Polarization horns appear on wave-resistivity measurements when the sensor approaches and crosses a boundary. Fig. 10 provides three computer modeling images that demonstrate a case in which a well approaches a conductive overlaying shale. The proximity to the cap rock results in a bright spot appearing clearly on the deepest-reading image. The bright spot is less pronounced on the medium-reading image, and it is barely noticeable on the shallowest-reading image. A look at all three images simultaneously provides a sense of depth of field and helps to indicate when a well is approaching a boundary (Diaz et al. 2009).

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Fig. 10—Computer simulation of images and bright spots from three spacings of the azimuthal deep resistivity tool.

3. Geosignals. The geosignal is used to provide a quantitative estimate of the DTB boundary. This estimate represents the magnitude and orientation of the “unsymmetrization” of the electromagnetic field created by the closeness of a bed boundary. Fig. 11 shows the relationship between the magnitude of the geosignal and the DTB boundary (Al-Mutari et al. 2009).

Fig. 11—Geosignal measures the lack of symmetry of the electromagnetic signal induced by a nearby boundary. The magnitude of the geosignal depends on resistivity levels and distance to geological boundary.

4. Distance-to-Bed Boundary. The DTB boundary calculation can be obtained with the interaction of the 3D geosteering software. Using the geosignals as entry variables, the distance from the wellbore to the top and bottom of the unit is calculated in feet or meters, as required, and saved in the database. This distance is represented in the visualization as blue or red signs, which represent either top or bottom proximity, respectively (Bejarano et al. 2010).

Interpreting Azimuthal Resistivity Data While Geosteering. Based on the geological scenario and the objectives proposed for the Alice 16H well, several variables were selected to be sent in real time with the optimum depth of investigation (DOI) (Table 1). This data would be entered in the correlation software to enable the following:

Provide sufficient reaction time and space in the event of an approximation into a non-desired zone (cap). Calculate the approximation to undesired boundaries (lithological in the case of this study), above and below the

well trajectory. Map the top of M1 SST reservoir aided with the DTB calculation. The permanence within the expected geological zone (zone with improved petrophysical characteristics), for

optimum fluid recovery

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TABLE 1—AZIMUTHAL RESISTIVITIES AND GEOSIGNALS PULSED IN REAL TIME

Resistivity Image 32 in.; 500 Khz DOI: 46 in.

Top/Bottom Resistivity 48-in.; 500 Khz (DOI:60 in.

Average Resistivity 50-in.; 500 long span DOI: 70 in. 82-in; 500 long span DOI: 95 in.

Geosignals 80-in.; 500 Khz. Phase DOI: 90 in.

112-in.; 500 Khz Phase DOI: 117 in.

DOI based on RT 100 Ohm-m and thickness of 30 ft. Geosteering Section To drill the horizontal phase the use of an Azimuthal Deep Resistivity tool as a proactive sensor in combination with a Gamma Ray tool as the reactive type Geosteering measurement for optimal well bore positioning was suggested to Andes Petroleum. Having as a reference the initial data obtained by petrophysical sensors while landing, was possible to observe a good correlation between real time information and calculated Gamma Ray and modeled Resistivity curves having as a reference Alice 12 well (offset). Critical Points Based on the initial correlations and the behavior of the azimuthal resistivity curves pulsed in real time (46-in. DOI image, 60 in. DOI resistivity Up/Down, and 90 and 117 in. DOI geosignals), the well was positioned in the reservoir at 3 ft TVD from the top (7,878.6 ft TVD) and slowly closing in on a lower resistivity zone above the reservoir; a structural low between 9,800 and 9,880 ft MD was projected.

Fig. 12—Computer-generated model of ALICE 16H well at 9,738 ft/7,711.5 ft TVD; beginning of the steering phase. Initial geological scenario and well plan. Following the model and the response of azimuthal data, drilling continued with 89 degrees of inclination while monitoring the top of the reservoir with a controlled ROP. A ROP range of between 50 and 70 fph was recommended to achieve an acceptable data density. A stated purpose was to map the top of the reservoir. According to Andes Petroleum Geology Department, the purpose was to map the top of the reservoir. At 9797 ft. MD (7879 ft. TVD), azimuthal resistivity responses indicated a constant approach towards the top of the reservoir. The polarization of the bottom resistivity readings plus an

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slightly increment on the geosingals magnitude towards positive, suggested the bit was located at the lowest portion of the structure at 2 ft in TVD from the top of the M-1 sandstone (value obtained from the DTB calculation).

Fig. 13—Computer-generated model of offset ALICE 16H well at 9,797ft/7,879 ft TVD projected.

At 9,900 ft MD, drilling with 90 degrees of inclination, a noticeable stabilization of the azimuthal responses was

observed. This stabilization indicated that the steering was parallel to the top of the reservoir in the lowest zone of the structure. The polarization of the resistivity and the formation of a clear bright spot in the resistivity image indicated a positioning very near (less than 1 ft TVD) the top of the reservoir. At 9,900 ft MD, some up-dip structural changes with +/-0.8 degrees of apparent inclination were noticed, as shown in Fig. 14.

Fig. 14—Computer-generated model of offset ALICE 16H well at 9,900 ft / 7,879 ft TVD projected.

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Minor issues related to the rising trend of the formation were reported at 10024 ft. MD (7879 ft. TVD) with at-bit-inclination angles higher than 91 degrees despite the steering effort in trying to drop the angle to the recommended inclination of 90 deg., while the azimuthal resistivity curves at this point showed an imminent proximity to the top of the M-1 SST. The azimuthal top bin resistivity was slowly dropping, which indicated that the tool was reading the low resistivity layer located above the reservoir due to the continuous rise in the drilling angle. An apparent dip of +/- 0.4 degrees was projected for the bed in this zone.

Fig. 15— Computer-generated model of offset ALICE 16H well at 10,024 ft MD.

Despite the directional drilling efforts to decrease the inclination angle to the recommended 90 degrees to maintain the TVD at 7,881 ft, the well inclination continued to rise. This situation was brought to the attention of the multidisciplinary team; the team considered the possibility of pulling out and continuing with a tri-cone bit to facilitate the operation in accordance to the recommendation. The team decided to stop drilling. The well reached a total depth of 10,050 ft MD / 7,878.22 ft TVD with an inclination angle of 91.5 degrees projected to the bit. Beginning at 9,474 ft MD through 10,050 ft MD, the drilled area included a lateral section of 576 ft of 100% M1 Sandstone, with very good characteristics.

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Fig. 16—Computer-generated model of offset ALICE 16H well at 10,050 ft MD. Conclusions The geology in the Oriente basin is complex; most of the reservoirs consist of small stacked channel sandstone sequences. For that reason, it is difficult to correlate reservoirs from well to well with limited lateral extent. Horizontal well placement in this kind of environment generally requires adjusting the directional well plan to change the navigation TVD.

A lateral section of 576 ft MD were drilled in the sweet spot of the M1 SST formation using the azimuthal deep resistivity sensor and a 3D interactive correlation software. After azimuthal resistivity information, combined with other basic petrophysical data, were compared, it was concluded that the reservoir top was 35 ft shallower than expected during the landing of the well Alice 16H. Having this as a departure point, the initial directional well plan had to be changed to optimize the trajectory to deliver a successful geosteering operation. Some minor adjustments along the 6-1/8-in. section enabled the proper geological placement of the well, and the accurate navigation contributed to geosteering the well into the best zone of the reservoir. Although the initial production of this well was estimated at 800 barrels of oil per day, its actual production is 6,800 barrels of oil per day. Previous economic expectations have been exceeded eight-fold, making possible the recovery of the originally forecasted investment of 84 days in just 11 days.

A large reduction of the geological and placement uncertainties was achieved by applying an efficient geosteering technique with azimuthal resistivity data available. Accordingly, new models and interpretations could be updated in real time in a relatively fast process and tied to existing pre-drill information, such as seismic and well logs within a truly dynamic-positioning process. Sudden lateral variations could be interpreted, as they began to influence the deeper resistivities, maintaining the well within the payzone for much of its lateral extension, despite the considerable TVD variations from the planned trajectory that were often necessary.

The use of a latest generation azimuthal resistivity tool and an integrated real-time modeling application, in conjunction with constant participation of the group of geoscientists and engineers in charge of drilling activities, absolutely proved that geosteering can be successful by improving positioning and reducing rig time, even under such complex scenarios. Acknowledgements The authors wish to thank Andes Petroleum and Halliburton for their permission to publish this information and the SPE committee for the opportunity to present this work. References Al-Mutari, B., Jumah, S., Al-Ajmi, H. et al. 2009. Geosteering for Maximum Contact in Thin-Layer Well Placement. Paper SPE 120551

presented at the SPE Middle East Oil and Gas Show and Conference, Bahrain, Bahrain, 15-18 March. Bejarano, C., Manrique, C., Chacin, L. et al. 2010. Challenging Complex Deltaic Reservoirs on the Orinoco Heavy-Oil Belt Aided by New-

Generation Azimuthal Deep-Resistivity Tools and Advanced Real-Time Geosteering Techniques: Successful Case Study from the Eastern Venezuela Basin. Paper SPE 139134 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Lima, Peru, 1-3 December.

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Bittar, M., Klein, J., Beste, R., Hu, G., Wu, M., Pitcher, J., et al. 2007. A New Azimuthal Deep-Reading Resistivity Tool for Geosteering and Advanced Formation Evaluation. Paper SPE 109971 presented at the SPE Annual Technical Conference and Exhibition, Anaheim, California, USA, 11-14 November.

Chemali, R., Bittar, M., Calleja, B., Hawkins, D., and Manrique, C. 2009. Azimuthal Wave Resistivity Opens a Window on the Geology Away from the Wellbore Path. Paper SPE 121894 presented at the SPE EUROPEC/EAGE Conference and Exhibition, Amsterdam, The Netherlands, 8-11 June.

Diaz, M., Iza, A., Rodas, J. et al. 2009. Successful Geosteering in Ecuador Using the Bright Spot Phenomenon from Deep Resistivity Images. Paper SPE 122794 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Cartagena, Colombia, 31 May-3 June.

Lesso, W., Kashikar, S. 1996. The Principles and Procedures of Geosteering. Paper IADC/SPE 35051 presented at the IADC/SPE Drilling Conference, New Orleans, Louisiana, USA, 12-15 March.

Muñoz, M., 2007. Optimización del Recobro de Petróleo en Yacimientos con Empuje Hidráulico Activo de Fondo Mediante la Perforación de Pozos Horizontales Utilizando la Herramienta de Geodireccionamiento y Completando con Mallas Para Control de Arena en el Campo Alice del Bloque Tarapoa. Project to apply for the degree of Petroleum Engineer. Quito, Ecuador.