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Abstract We have characterized the response of a compact 2-inch (57mm) diameter open-hole formation density logging tool for cased hole environments. Data are processed with an established open hole transform in which the casing effect appears as a simple attenuation term in the count rate domain, and variations in cement thickness are compensated using a classical dual-detector spine-and-ribs approach applied in the density domain. The combination of through-casing density and casing-corrected neutron porosity has been applied to the evaluation of by-passed pay and shallow gas, and to the evaluation of wells where open hole acquisition has not been feasible for operational, hole quality, or economic reasons. The tool-specific neutron porosity excavation effect has been characterized for gas-bearing sands.

Case history and model results suggest accurate formation densities are achievable for casing thicknesses up to about 0.35 inches (9mm) if cement is less than about 1 inch (25mm) thick, albeit with loss of precision relative to open hole. Formation sensitivity declines with increased cement thickness until practically all is lost for casing standoffs in excess of 1.5 inches (38mm). For modest thicknesses, however, cased hole density-neutron gas evaluation has advantages relative to the neutron-dipole sonic method; in particular it does not rely on good cement bond, it has generally superior spatial resolution, and (optionally) data can be acquired in memory mode on slickline in operating environments that do not favour conventional wireline units.

Introduction The present work was stimulated by the search for shallow gas, and has since found broader application in the evaluation of by-passed pay (including light oil), in the monitoring of fluid contacts and saturation changes over time, and in the general evaluation of intervals that, for a variety of reasons, may not have been logged open hole.

Gas behind casing can be inferred from pulsed neutron tool

count rate logs, and the 111/16 inch (43mm) diameter variants may be the sole evaluation option in wells with small diameter tubing restrictions. A common alternative in other cases (and where pulsed neutron tools are precluded by other operational or cost issues) is a dual porosity approach with neutron and sonic curves scaled empirically to overlay in clean water-bearing zones. Sonic porosity measurements have the advantage of being insensitive to hole enlargement behind casing, but quality control of waveform processing is time-consuming, and the velocity-porosity transform may be uncertain. In poorly bonded casing the formation arrival may be lost altogether. A more technically challenging third option is the density-neutron combination. This is feasible where casing thickness and standoff are modest, but is demanding because of the need to deal with variable casing standoff.

Density tools respond to casing as though it were heavy mudcake, except measured density values are typically high and beyond the range for which tools are normally characterized. When this is compounded with unknown casing standoff, standard processing yields inappropriate values of delta-rho and inaccurate compensated densities.

One approach to cased hole density correction is to normalize the compensated density log in zones of known porosity, or align to neutron porosity logs in clean water-bearing intervals (Cigni and Magrassi, 1987). This can be adequate for delineating zones of interest, but is unlikely to furnish good accuracy over a broad range of density values, and provides little or no information to allow variations in casing standoff to be flagged or accounted for. A better approach is to correct individual apparent density measurements (Near and Far curves in the case of a dual detector device), but it is important to recognise that measured and actual densities may not be linearly related outside the range of densities for which the tool is normally calibrated.

Among the factors affecting measured density in cased wells are casing thickness, casing density, cement thickness and cement density, as well as formation density. To account fully for all these variables implies a requirement for multiple independent density measurements with different depths of investigation. Laboratory results from an experimental four-detector device have been published (Moake, 1998), but there is no evidence of the tool being run commercially. Field experiences with a conventionally-sized density tool processed with laboratory-derived cased hole calibrations have also been reported (Ellis et.al., 2004). The present work relates to a commercial dual-detector tool, and is an extension of a

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A Novel Cased Hole Density-Neutron LogCharacteristics and Interpretation P.A.S. Elkington, SPE, C.A. Pereira, and J.R. Samworth, Weatherford

methodology that assumes casing properties and cement density are constant in the interval analysed (Samworth and Calvert, 2002).

The tool is 2-inch (57mm) in diameter, developed for open holes but capable of being deployed into casing and production tubing. It is run on wireline for real-time surface readout, but in battery-memory mode it can be conveyed on slickline, drillpipe or coiled tubing (Elkington et. al., 2000). It is often run in combination with other 2-inch diameter tools from the same family, including neutron porosity and sonic. The ability to acquire open and cased data with the same tools has operational and evaluation advantages, particularly in monitoring applications that use open hole logs as the baseline.

The skid that contains the source and detectors is typically faced with an 8-inch (203mm) curvature wear plate, but for this study we used a 4-inch (102mm) profile plate developed originally for slim open holes, and which also provides a better fit to casing and tubing with internal diameters less than 6-inches (152mm). The skid is able to move in and out of a carrier, and it is reasonable to assume that it is in intimate contact with the casing at all times i.e. tool to casing standoff is assumed zero. The residual effects of well fluid density and casing diameter are small, and accounted for within the processing.

The cased hole density analysis starts with the single detector count rate transform. We derive the casing attenuation factors before moving to a dual detector compensation scheme. These factors were initially derived empirically, and a limited database of values was established. Monte Carlo modelling subsequently supported the empirical observations, and has helped to quantify sensitivities to key variables, and extend the range of application. We also show how cased hole density and neutron logs can be combined to provide quantitative porosity and saturation estimates in gas-bearing sands, and give examples of its application.

Cased Hole Density Response We derive formation density in two stages: transformation of count rates into apparent density values for each detector (the single detector response), followed by computation of formation density from a combination of apparent densities (the dual detector response). The process is identical to that employed in open hole, the only difference being the values assigned to certain key variables. Single Detector Response

In the single detector response model (Samworth, 1992), count rate, I, is related to formation density, , by: I = Ee-k + F(c-t)(me-km - e-k) + G (1) where m is borehole fluid density, c and t are hole and tool profile diameters respectively, and E, F, G and k are constants. E is a transmission term, and F and G relate to contributions from the borehole and tool respectively. F is of minor significance in open hole, being about four orders of magnitude less than the nominal value of E, but becomes more significant as casing thickness increases (and count rates decrease). G arises principally from internal low-activity 137Cs sources used in the tools gain stabilization system.

Figure 1 shows count rate vs. density characteristics when the borehole term is zero. It is similar to the commonly used linear log-normal characteristic over the range of typical reservoir rock densities, but is a more realistic representation of the actual response at high apparent densities (such as low porosity formations in cased wells), as well as at low apparent densities (such as coals in open holes). It also conforms to the boundary condition (violated by the linear log-normal form) of zero non-tool counts for density values of zero and infinity. The low density response is largely irrelevant in cased wells, but a good description of the response at high apparent densities simplifies the casing correction.

The count rate transform was investigated with the MCNP Monte Carlo modelling code. We looked at casing thicknesses of 0.10, 0.14, 0.20, 0.25, 0.35, 0.5 and 0.75 inches (2.5, 3.5, 5.0, 6.4, 8.9, 12.7 and 19mm) with zero cement thickness and formation densities of 2.65, 2.40, 2.16, and 1.66 g/cc. The formation part of equation (1) can be made to fit the count rate distribution for each casing thickness simply by modifying the E term (while making no change to k). This implies that the casing is attenuating the count rate in a straightforward way. The degree of attenuation is shown in Figure 2; it shows that the logarithm of the scaling factor applied to E varies linearly with casing thickness, reducing the Near and Far spacing values to 38% and 43% of their respective open hole levels for a casing thickness of 0.25 inches (6.4mm).

Fig. 1 Count rate vs. density response.

Fig. 2 Attenuation factor vs. casing thickness.

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The leading term in equation (1) now becomes: I = E e-mt e-k (2) where t is casing thickness and m the slope of the line in figure 2. Dual Detector Response Standoff effects for dual detector tools in open hole are commonly dealt with by the well-known spine-and-ribs technique. The method is valid even when the density of the material in front of the tool exceeds that of the formation itself, so might therefore be expected to work in cased wells. In practice, however, implementations are tuned to work with realistic mudcake densities and thicknesses, not high density steel casing (approximately 8 g/cc). Apparent densities measured by each detector are influenced by casing, cement and formation densities, and are typically above the range for which most tools are characterized. Therefore we do not expect standard open hole processing to succeed in cased wells.

There is another important consideration in respect spine-and-ribs processing that relates to precision and repeatability. In the limiting case of large standoff, ribs re-join the spine at a point corresponding to the density of the material between tool and formation figure 3. The formation density of any point along a rib is found by projecting the point back along the rib to the spine at the zero standoff condition, but small variations in the log density values at extreme standoffs have a large lever effect on the projection, causing the compensated result to oscillate wildly. For this reason, real-world implementations of spine-and-ribs constrain the projection i.e. they sacrifice some accuracy for improved precision. This is a particularly important consideration in cased holes where reduced count rates reduce statistical precision.

The single detector casing corrections remove casing effects on the individual Near and Far measurements so that the points are both closer to the spine and within the normal range of density values for which the tool is calibrated.

Having determined the E attenuation factor for each detector, it is assumed that any residual differences between Near and Far apparent density values are due to variations in cement thickness and/or cement density. In general there are

insufficient degrees of freedom with a two detector system to solve uniquely for both cement parameters, but if cement density is known independently we can still derive formation density using spine-and-ribs processing (after casing correction the ribs meet the spine at formation and cement density values).

In many cases, however, we can avoid the need to know (or solve for) cement density by using a linear rib assumption. Samworth (1992) has shown that within the classical spine-and-ribs framework, the density of the material between tool (or casing in this instance) and formation has no influence on the compensated density over the linear part of a rib (that part close to the spine).

Although real ribs are curved (because the rates at which formation sensitivity declines as standoff increases are different for each detector), the linear rib assumption is reasonable for modest standoffs, and for reasons outlined previously, this also has an advantage in respect of precision.

With these insights, a casing corrected density is readily computed by substituting equation (2) into the formation part of equation (1). The remaining process is identical to that for open hole, producing a compensated density, C, and degree of compensation, . The latter is computed in the usual way, namely: = C F (3) where F is the corrected Far density. reflects the degree of casing standoff (cement thickness), and may be larger than in open hole where standoff due to mudcake is typically less.

The method does not correct for the additional steel thickness at casing collars, and it is important to review results alongside a CCL log. The C curve tends to read light opposite collars because they cause the Near to read heavier than the Far, making negative. Collar effects can be mitigated by suppressing the Near contribution over collar intervals.

Support from Empirical Evidence The model-derived attenuation factors applied to the Near and Far E values in cased wells can be verified empirically by examining distributions of Near and Far density values in real well environments.

Figure 4 is a crossplot of values obtained with standard open hole processing in a sand-shale-coal sequence in 4.5-inch (114mm), 9.5 lb/ft (14 kg/m) casing in a 6.25-inch (159mm) well. Migrating this data back to the spine using open hole ribs produces grossly inaccurate compensated densities. If we have a-priori knowledge of at least one formation density value, however, the data can be moved to the position it would occupy on the crossplot if there were no casing (nominal casing standoff needs to be assumed). We would then expect to see all non-collar points distributed above the spine in a manner that reflects cement standoff only.

Linear shifts are unsatisfactory (because high apparent density values may not be linearly related to actual densities), but using equations (1) and (2) it is possible to determine the attenuation factors that move the data to match the density of the known point. Figure 5 shows the same data corrected in this way. Note that the data cluster approaches the spine at two points: at high formation densities, and at the density of the cement (about 2 g/cc). The lowest density values are coals, and these appear below the spine after correction. Attenuation

Fig 3 Uncertainty in compensated density at large standoffs, no casing corrections applied. As Near and Far log values approachthe high apparent density point, uncertainty in the projectedcompensated density value increases.

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factors inferred in this way are consistent with those from model results.

Efficacy of the cement thickness compensation may be judged by comparing passes acquired with the density tool in different azimuthal orientations, and processed in two different ways (cement thickness is assumed to vary around the casing except where it is centralized in a circular section hole). Figure 6 shows two such passes, one processed by shifting Near and Far linearly, the other using the count rate model and non-linear shift. The latter has the better repeatability between main and repeat passes. The greatest difference is in the interval 555m - 562m where differs between runs (indicating the tool azimuth was different between runs); linear processing does not correct the difference, whereas count rate processing does.

Density logs processed in this way are consistent with offset open hole data and other porosity logs where casing and cement thicknesses are modest. Quality reduces as thicknesses increase, with practical limits being in the region of 0.5 inches (13mm) casing thickness and/or about 1.5 inches (38mm) cement thickness. This is consistent with model results that show, for example, that for a casing thickness of 0.35 inches (9mm) the Far density has usable sensitivity to a cement thickness of about 2 inches (51mm), but for the Near the limit is about 1 inch (25mm).

Fig 6 Overlay of main and repeat pass cased hole density logs.Upper log Near and Far shifted linearly. Lower log Near and Far processed via the count rate model and non-linear shift. Density scales are 1.9 to 2.9 g/cc; -0.05 to 0.45 g/cc over the rightmost 5 grid lines; gamma ray logs and CCL in the left track.Note the interval 555m - 562m where main and repeat passes agree after processing with the count rate model. Fig. 5 Near and Far density values after cased hole processing.

Fig. 4 Near and Far density values before cased hole processing.

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Cased Hole Neutron Response Gas bearing formations are classically delineated in open hole using a density-neutron combination, and with appropriate corrections, the same can be true of cased holes. Indeed, formations that have produced gas after casing may have a stronger gas signature relative to the open hole situation (in which the gas signal has been suppressed by fluids invasion). It has also been suggested that count rate ratios (from which porosity is derived) are more accurate predictors of gas saturation than sigma derived from pulsed source tools (Cowan and Wright, 1999). In this section we examine the neutron cased hole response, and in particular we look at the effects of gas saturation on the density and neutron responses behind casing.

Cased hole corrections for the 2-inch (57mm) neutron tool have been available for some time figure 7. They are defined by three parameters - casing ID, casing thickness, and cement thickness. These replace the borehole size and standoff corrections required in open hole (the casing itself does not have a large effect on the neutron flux around the tool, cement having the greater influence). Corrections determined for each parameter sum to the net correction. In the case shown, apparent neutron porosity in 0.5-inch (13mm) thick, 5-inch (127mm) ID casing with 1.25-inch (32mm) thick cement is 28 pu, the individual corrections are 3.9, -7.5 and -6.6 pu, and corrected porosity is 17.8 pu. Efficacy is illustrated in figure 8, which compares the same interval logged open then cased hole.

Gas Effect Gas in the formation reduces apparent neutron porosity. This so-called excavation effect has been described for an older tool (Segasman and Liu, 1971). We used MCNP to model the effects of gas saturation on the compact tool neutron response, taking into account typical temperature/pressure gradients. Gas composition was assumed to be methane, and water was assumed fresh; temperature and pressure effects on water and gas densities were computed from equations developed by

Batzle and Wang (1992). Results are referred back to the standard condition of 8-inch diameter open hole.

Apparent log densities were computed from formation electron densities using the inverse Z/A relationship for this tool (Samworth, 1992). In order to allow application to field porosity logs computed assuming full water saturation and a fixed value of filtrate density, we have converted measured density values to apparent density porosities using a nominal filtrate density value of 1.0 g/cc. Results for 2,000 psi/50C (13.8 MPa/122F) are plotted on a conventional density-neutron crossplot in figure 9. Constant porosity lines are extensions of the limestone to sandstone equi-porosity lines, and lines of constant saturation are very approximately orthogonal to them. Plotted on a dual-porosity crossplot, the constant porosity and saturation lines are linear. Results for 7,000 psi and 120C (48.3 MPa / 248F) are plotted in Figure 10. Example

Figure 11 is a through casing density-neutron porosity and gas saturation analysis from a high-value horizontal well in which no open hole density-neutron data were acquired due to failure of an LWD string. Conveying wireline tools on coiled tubing was considered, but it was not possible to use a coil with integral wireline due to weight restrictions. The solution was to deploy the compact density-neutron in battery-memory

Fig.7 Neutron porosity cased hole corrections.

Fig. 8 Neutron porosity comparison. Green was acquired in open hole, red in cased hole. Scale is 60 pu (left) to 0 pu (right).Left track shows open and cased gamma ray(uncorrected), andCCL. Casing is 7-inches, 21.5 lb/ft in an 8.5-inch bit size well

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Fig. 9 Gas saturation on density-neutron crossplot.

Fig. 10 Gas saturation on dual porosity crossplot. Fig. 11 Porosity and gas saturation analysis.

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mode on the wireless coil. Nominal casing and cement thicknesses are 0.27 (7mm) and 0.75 inches (19mm) respectively.

Correcting both the density and neutron for casing reveals that gas is confined to numerous thin intervals. Corrected porosity plots between the density and neutron where there is gas crossover, and saturation is shown in track 1. Separation of Near and Far neutron count rates provide corroboratory evidence for gas. Comparison with Sonic-Neutron for Gas Detection The processing has been applied to dozens of cased wells, and interpretations compared with those from the sonic-neutron method. Figure 12 is an overlay of through casing sonic and density in a sand-shale-coal sequence. The sonic comes from semblance processing, and even though cement bond is good in this interval, it lacks the resolution of the density. Alternative processing can recover some resolution, but it is the density whose character is most similar to that of the neutron porosity.

Summary and Conclusions The key to deriving formation density through casing from a dual detector tool is appropriate individual processing of the Near and Far curves to remove the effect of casing thickness; spine-and-ribs compensation can then used to correct for the

effects of variable casing standoff. As standoff increases, small variations in measured density translate to increasingly large changes in compensated density, but precision can be maintained at the cost of some reduction in accuracy if linear ribs are assumed. Linear ribs have the additional property of not requiring explicit knowledge of cement density.

The combination of density and neutron through casing provides an attractive gas indicator. Modelling has shown that gas saturation has a straightforward effect on the neutron response, and that porosity and saturation can be computed from the dual porosity combination using simple linear equations. This is a useful alternative to the sonic-neutron dual porosity approach.

Acknowledgements The authors are grateful to Weatherford Evaluation, Drilling and Intervention Services for permission to publish this paper. References

Batzle, M. and Wang, Z. (1992). Seismic properties of pore fluids.

Geophysics v 57, p 1396 - 1408 Cowan, P. and Wright, G. A., 1999. Investigations into improved

methods of saturation determination using pulsed neutron capture tools. SPWLA 41st Annual Logging Symposium, paper P.

Cigni, M., and Magrassi, M., 1987. Gas detection from formation density and compensated neutron logs in cased hole, SPWLA 28th Annual Logging Symposium, paper W.

Elkington, P. A. S., 2000. The role of open hole memory logging and wireless conveyance systems in the evaluation of horizontal wells, paper SPE 65461, presented at the SPE/Petroleum Society of CIM International Conference on Horizontal Well Technology, 6-8 November 2000.

Ellis, D., Lling, M., G., Markley, M., E., Moss, L., Neumann, S., Pilot, G., and Stowe, I., 2004. Cased hole formation density logging some field experiences, SPWLA 45th Annual Logging Symposium, paper G.

Moake, G.L., 1998. Design of a cased hole logging tool using laboratory measurements, paper SPE 49226, presented at the SPE 73rd Annual Technical Conference.

Samworth, J. R., 1992. The Dual Spaced Density Log Characteristics, Calibration and Compensation, The Log Analyst, vol. 33, n.1, p. 42-49

Samworth, J.R., and Calvert, S., 2002. Differentiation of reservoir fluids through casing an application of the compact memory logging technique, SPWLA European Formation Evaluation Symposium, London, September 5th - 6th.

Segasman, F., and Liu, O., 1971. The excavation effect, SPWLA 12th Annual Logging Symposium, paper N.

SI Metric Conversion Factors in. x 2.54* E+00 = mm psi x 6.894 757 E+03 = MPa (F*- 32)/1.8 E+00 = C *Conversion factor is exact.

Fig. 12 Cased hole density and sonic porosity overlay. Track 1: Gamma ray (green) and CCL (blue). Track 2: Density (red), sonic (blue) 60 to 0 pu.

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