7/21/2019 SPE-116528-MS

1/14

SPE 116528

Horizontal Well Best Practices to Reverse Production Decline in MatureFields in South China SeaYou Hongqing, Wei Ping, Tian Xiang, Xu Xiang Dong, Lian JiHong, Thanh Tran, Yoseph J. Partono : CACT,Jeffrey Kok, Liu Yang, Sarfraz Balka: Schlumberger

Copyright 2008, Society of Petroleum Engineers

This paper was prepared for presentation at the 2008 SPE Asia Pacific Oil & Gas Conference and Exhibition held in Perth, Australia, 2022 October 2008.

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 beenreviewed 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, itsofficers, 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 toreproduce 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.

AbstractThe Huizhou 6S and 3S oil fields in the Pearl River Basin,

Offshore South China Sea are mature fields which have

produced 40% to 60% of their original oil in place since1991. Currently the field production is rapidly declining

and water production is increasing. However, through

reservoir surveillance data, geologic and reservoirmodeling, significant recoverable oil was identified in

shaly sandstone reservoirs and attic structural locations of

clean sandstone reservoirs. As a result, a comprehensiveportfolio of prospects has been built for a robust

development program. Horizontal wells were utilized toimprove oil recovery in shaly sands and to reduce water

coning in thin remaining oil columns. Horizontal drillingbest practices were applied during well planning and

drilling executions, such as optimum well designs,specific LWD/MWD tool selections, low fluid loss

drilling fluids, real-time geosteering data monitoring and

the cleaning of the pay zone during completions were

applied to maximize reserve recovery and successfullyreverse the fields production decline.

IntroductionThe Huizhou Oil Fields are located in the Pearl River

Mouth Basin, South China Sea approximately 190 kmsoutheast of Hong Kong (Figures 1). Agip, Chevron and

Texaco signed the contract in 1983 and they discovered

nine prolific oil fields by the year of 1989. Firstproduction in this area was from the Huizhou 1S field in

1990 followed by Huizou 6S field in 1991 and Huizhou

3S field in 1995. There are currently 7 platforms and anFPSO which are producing oil and gas from 10 fields.

New efforts were put in the exploration activity with the

drilling of exploration and appraisal wells in the period of2002-5resulting in another significant discovery and the

commercialization of two previously discovered fields.

Geological OverviewHuizhou 6S and 3S oil fields are located in Pearl River

Mouth basin, South China Sea, which is a continental

margin sedimentary basin formed during the rifting of theSouth China Sea in the Late Mesozoic to early Tertiary.

The main pay zones where CACT deploys horizontal and

multilateral well technology were deposited as delta frontbar and coalesced, stacked fluvial-deltaic channel sands in

early Miocene time. Oil accumulations with low gas

saturation and no gas cap are mostly found in 4-way-dipclosures, which are associated with basement highs. In

some cases the traps are also associated with sand layerpinch-outs.

The reasons for drilling horizontal wells arepredominantly based on the following two considerations:

Firstly, after over 15 years of high rate oil production,these fields have gradually entered into a mature and

high-water-cut phase. Oil to water contacts (OWCs) have

significantly encroached upward leaving thin remaining

oil columns and causing high water production from all ofthe existing wells. Due to reservoir heterogeneity, some

of the reservoirs exhibit uneven strength of aquifersupport causing OWCs to tilt after years of production.

To reduce and reverse the rapidly declining oil production

rate is a challenge CACT needs to confront. Tosignificantly improve production efficiency of theremaining potential attic oil locations, horizontal well is

the first priority among various alternatives.In addition, the poor petrophysical properties of shaly

layers did not effectively produce oil from vertical wells.

The recent horizontal and multilateral wells in these shaly

zones have greatly improved production performance andfield recovery. The petrophysics and depositional

environment of the reservoirs are briefly described asfollows.

7/21/2019 SPE-116528-MS

2/14

7/21/2019 SPE-116528-MS

3/14

SPE 116528 3

in vertical depth which could be detrimental to

the success of a horizontal well drilling and

completion. This risk may be mitigated bydrilling a pilot hole prior to drilling the

horizontal well. In May 2008, CACTreprocessed the 3-D seismic data using a pre-

stacked depth migration (PSDM) technique to

reduce the structural risk.- The risk of not finding sufficient oil

accumulation because the current OWC have

moved higher than expected leaving too thin oilcolumn to produce. If there is no recent

information on the current fluid contact within

the radii of 1000m, it is important to acquire data

to locate the current OWC. Cased hole loggingin nearby wells is a low cost option to survey the

current fluid contact. If reliable cased-hole datacannot be acquired, a pilot hole is to be drilled

prior to drilling a horizontal well.

To anticipate the possibility that the pilot hole does notproof sufficient reserves in the target reservoir due to the

above risks a backup target (Figure 5) is prepared as acontingency against drilling an uneconomic horizontal

well.

Pre-drilling PreparationOnce a horizontal target is selected a detailed well course

is planned to ensure successful placement of thehorizontal section in the reservoir, casing run, casing

cementation and installation of completion.

1. Optimum Well Plan

The design of the well plan requires close collaboration

between geologist and drilling engineer in order to ensurethat the well trajectory is drillable and to reduce risks tothe lowest level and to achieve the best performing well.

The direction of the existing well to be sidetracked does

not usually following the simplest route to reach the targetlocation because of proximity with other existing wells.

This condition causes difficulty in achieving low well

tortuosity and dogleg severity, where iterative targetadjustments are required to optimize the well plan and

ensure the modeled torque and drag are within allowable

limits.A tangent section of 5m true vertical depth (TVD) above

the target is generally planned to accommodate

uncertainties in the geologic structure interpretation anddirectional survey. (Fig 6)

Landing point of the horizontal section is typically

planned 1m or less below the top of the reservoir to allowmaximum displacement from the current OWC. The

angle along the horizontal section should not exceed 90

degrees to avoid a goose-neck trajectory that could

cause water blockage during production. Depending onthe quality of the reservoir and the simulation results a

horizontal section of 300 to 1000m are planned.To accommodate difficult well drilling operations the

following upgrades were done on the platform and

drilling equipment:

1- 225 tons pipe deck and 3500 m new derrick

2- Hoisting rating: 350 klbs3- Hydraulic TDS: 37,000 ft-lb

4- Mud pumps: 2 x P-750 + 1 x 3NB-800

5- Mud tank: 900 bbls including one removabletank

6- Shale Shakers: 3 X Derrick Hydrill2000 with

0.47 cubic feet per min for each7- Centrifuge: 1 X Swaco518FVS

Well Profil e

All the well profiles were designed in 3-D, (Fig 7). In

order to conventionally hit the planned target TVD,

medium to high doglegs from 3.5 to 4.5 are required.

But it was possible to limit doglegs to the minimum bydropping the inclination with a turn first and then building

in the other direction. While designing the well profile, atangent section for ESP and holding of inclination from

85-87 for a soft landing were also key requirements.

The average open hole length for the sidetrack wellvaries

between 2000 to 2500m. The 3-D profile designs producehigh tortuosity and high Directional Difficulty Index (Fig

8 and Fig 9), where the average tortuosity can varybetween 220-250. Using a water base mud system

further increased the torque and drag issues. To over-

come this, basic drilling surface parameters such as RPM,

WOB, mud pump flow rates are controlled. In addition,well bore improvement techniques such as adding

lubricants to lower friction factors and the usage of rotarysteerable system (RSS) were employed. The application

of RSS tool delivered excellent drilling results in all the

wells, with the exception of one case where the tortuosity

increases above 250 deg that created some issues whenrunning a 7 liner.

Depending on the reservoir properties, multilateral wellswere also planned. The side track point selection was akey element of success that required detail planning and

execution by the drilling and reservoir teams. In total,

three multilateral wells were successfully drilled andcompleted with the combined teamwork and proper BHA

selection.

2. Technology Selection

Whipstock and mi lli ng Window: In many occasions there

is little cement behind the casing where a sidetrackwindow is milled. This leaves a sharp, off-sized window

that causes problems when a drilling BHA or casing

packer is passed through. As a remedy, a cement squeezearound the window is performed, giving very positive

results. Experiences from other parts of the world showed

that after the milling operation, the drilling BHAs had thetendency to track the casing. To address this problem, the

whipstock on CACT wells are set either left or right of the

high side.

Bi t Selection: Historically in CACT, drilling with water

base mud (WBM) in abrasive formation with the presenceof hard and interbedded stringers greatly reduces a PDC

bits performance with bit balling and premature broken

7/21/2019 SPE-116528-MS

4/14

7/21/2019 SPE-116528-MS

5/14

SPE 116528 5

hole and modeling the liner run with various simulations

to determine the number and position of liner centralizers

required.With this practice, all the wells except one underwent

very good results with the liner run arriving to bottom. Inthat single case (well HZ6S-10Sb), the liner got

prematurely stuck leaving almost 400m of open hole

exposed. Further investigations identified the root causeof such sticking to the liner hanger assembly. As aremedial, a rotating liner hanger was implemented for

future liner designs to help break down the excessivefriction by liner rotation and push the liner to bottom.

ConclusionsHorizontal and multilateral wells have proven effective to

produce remaining oil in thin columns and shaly

formations in mature HZ3S and HZ6S oil fields.

Current surveillance data and pilot holes are essentials toprovide controls for drilling horizontal wells.

Vigilant team work between subsurface, well placement

and drilling teams during all phases of operations,including reservoir modeling, well planning and well

drilling was necessary to provide strategic planning with

contingencies for the best and timely real time decisions.The sidetrack campaign on the HZ3S and HZ6S fieldsin

2007 successfully reversed the production decline wherethe cumulative oil in 2007 from four sidetrack wells

totaled around 1.5MMSTB. This accounted for 30% of

the total field production where the oil rates from these

four wells held 30% of the total field production.

AcknowledgmentsThe authors would like to thank CACT, CNOOC, ENIand Chevron for the permission to publish this

information.

AbbreviationLWD: Logging while drilling

MWD: Measurement while drillingFPSO: Floating Production Storage & Offloading

OWC: Oil Water Contact

BOPD: Barrels of Oil per Day

PVT: Pressure-Volume-TemperaturePSDM: Pre-Stacked Depth Migration

TVD: True Vertical DepthTDS: Top Drive System

ESP: Electric Submersible Pump

RPM: Revolutions per MinuteWOB: Weight on BitRSS: Rotary Steerable System

PDM: Positive displacement mud motor

BHA: Bottom Hole AssemblyPOB: Personnel On Board (Rig or Platform)

WBM: Water Based Mud

PDC: Polycrystalline Diamond CompactROP: Rate of Penetration

DIF: Drill in Fluid

API: American Petroleum InstituteTD: True Depth

RT: Real Time

RTGS: Real Time Geosteering Software

DLS: Dogleg Severity

CD&I: Continuous direction & inclinationMMSTB: Million Standard Barrels "oil"

7/21/2019 SPE-116528-MS

6/14

6 SPE 116528

Figure 1: CACT oilfields location map

7/21/2019 SPE-116528-MS

7/14

SPE 116528 7

Figure 2: Generalized stratigraphy of Pearl River Mouth Basin.

7/21/2019 SPE-116528-MS

8/14

7/21/2019 SPE-116528-MS

9/14

SPE 116528 9

Figure 5 Cross section showing a planned horizontal well with a pilot hole and a backup horizontal target

7/21/2019 SPE-116528-MS

10/14

10 SPE 116528

Fig 6: Depth Uncertainties in Landing a Horizontal Well

Fig 7 Well Profile

7/21/2019 SPE-116528-MS

11/14

SPE 116528 11

Fig 8 Torque and drag modeling, Tripping load analysis will be required

Fig 9 Torque and drag modeling, Tripping load analysis will be required

7/21/2019 SPE-116528-MS

12/14

7/21/2019 SPE-116528-MS

13/14

SPE 116528 13

ADN475 RT Image on InterAct

Layer 1entr

Cutting up strat

Fig11 Density Image Example

7/21/2019 SPE-116528-MS

14/14

14 SPE 116528

Fig 12 Landing Model and Correlation Example

Fig 13 Horizontal Section