No abstract

Summary Permeability is one of the key petrophysical parameters in reservoir evaluation. Information about permeability is commonly derived from cores and test data, which generally cover only part of the reservoir section, but can also be derived from logs and then extrapolated to uncored intervals and wells. Two logs provide such information: acoustic and nuclear magnetic resonance (NMR). In the Karachaganak field, an approach based on the acoustic tool was preferred because of the textural characteristics of the vuggy carbonate reservoir. The approach relies also on the use of image logs to obtain a detailed description of the reservoir-rocks texture and to discriminate between rocks with primary interparticle porosity or very small vugs and lithotypes with multimodal distribution of pores, enlarged vugs, and touching vugs. More than 900 m of core have been used to validate the permeability log derived from the analysis of Stoneley waves in 25 wells from this field. A correlation between the validated log-derived permeability and the textural facies from image logs has allowed the relationship between permeability variations and the geological framework to be established. The results have been compared with dynamic data from production logging through the definition of flow units from Stoneleywave-derived permeability and porosity-log data and the use of a stratigraphic-modified Lorenz plot (SMLP) to identify possible fluid-entry points. Three main permeability trends have been identified: For undolomitized or patchily dolomitized biohermal deposits for pervasively dolomitized lithologies characterized by lower mean permeability values for facies characterized by well-developed vuggy porosity with enhanced dissolution phenomena (i.e., touching vugs and microfracturing) Also, well-test results have highlighted the occurrence, in some wells, of very high k values in the biohermal deposits, which are clearly unmatchable by log-derived matrix permeability. Open fractures have been observed in these wells on the image logs, thus suggesting that the enhanced permeability and the improved well performances have to be related to their presence. The comparison of log-derived permeability and well-test results has given a new perspective in the interpretation of well-test results with respect to the geological framework and a well-defined sequence-stratigraphic model. The extension of this methodology to new wells could improve knowledge of the petrophysical characteristics of the reservoir and allow better prediction of reservoir productivity.

The Abiod formation is the principal target in the Miskar field, offshore Tunisia. Consisting of fractured geomechanically stressed carbonate with a measured matrix permeability as low as 0.1 mD. The formation dates from Campanian to lower Maastrichtian and forms a horst structure. The formation has been under production since 1996. Obtaining formation pressure data was considered critical for determining the magnitude of depletion from production, well-to-well comparisons for vertical and lateral connectivity, forward modeling, completion decisions, and refinement of the field development plan. Historically, this has been a challenge with conventional wireline (WL) formation testers for the following reasons: Severe depletion and well deviation causing differential stickingHigh temperatures (150 to 195° C) at the limit of tool electronicsLow permeabilityFractures and breakouts that can impact seal success This was overcome with a systematic multidisciplinary approach. After review of historical formation testing data, and influence on seal success with probe vs. packer elements, it was decided to apply formation-pressure-while-drilling (FPWD) technology. The key questions with FPWD in this environment are: Can we achieve a good transient profile and what is potential impact of supercharging? These questions were addressed with advanced prejob modeling, which enabled determination of an optimized pretest configuration and testing procedure to minimize potential supercharging effects. While drilling, stage-in procedures were used, and mud logging total gas data were gathered to identify areas of liberated gas. Pre-run wireline petrophysical data were gathered to characterize the Petrophysical properties of the reservoir and to calculate an intrinsic permeability profile. Ultrasonic borehole images and caliper data were used to determine the principal horizontal stress directions, fracture frequency, and orientation and to confirm the stratigraphic dipping of the structure. Combined, this information allowed a focused orientation of the FPWD probe and optimal station selection avoiding fractures and breakouts. This novel approach resulted in 100% seal success, >50% improvement. Four days of rig time were saved, and the required data were obtained.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractPermeability is one of the key petrophysical parameters in reservoir evaluation. Information about permeability is commonly derived from cores and test data, that generally cover only part of the reservoir section, but can also be derived from logs, and then extrapolated to uncored intervals and wells. Two logs provide such information: acoustic and nuclear magnetic resonance. In the Karachaganak Field, an approach based on the acoustic tool was preferred because of the textural characteristics of the vuggy carbonate reservoir. The approach relies also on the use of image logs to obtain a detailed description of reservoir rocks texture and discriminate between rocks with primary interparticle porosity or very small vugs and lithotypes with multi modal distribution of pores, enlarged and touching vugs. More than 900 meters of core have been used to validate the permeability log derived from the analysis of Stoneley waves in 25 wells from this field. A correlation between the validated log derived permeability and the textural facies from image logs, has allowed the relationship between permeability variations and the geological framework to be established. The results have been compared with dynamic data from production logging through the definition of flow units from Stoneley wave-derived permeability and porosity log data and the use of a Stratigraphic Modified Lorenz Plot to identify possible fluid entry points. Three main permeability trends have been identified:• for undolomitised or patchily dolomitised biohermal deposits.

The Abiod formation is the principal target in the Miskar field, offshore Tunisia. Consisting of fractured geomechanically stressed carbonate with a measured matrix permeability as low as 0.1 mD. The formation dates from Campanian to lower Maastrichtian and forms a horst structure. The formation has been under production since 1996. Obtaining formation pressure data was considered critical for determining the magnitude of depletion from production, well-to-well comparisons for vertical and lateral connectivity, forward modeling, completion decisions, and refinement of the field development plan. Historically, this has been a challenge with conventional wireline (WL) formation testers for the following reasons: Severe depletion and well deviation causing differential sticking High temperatures (150 to 195° C) at the limit of tool electronics Low permeability Fractures and breakouts that can impact seal success This was overcome with a systematic multidisciplinary approach. After review of historical formation testing data, and influence on seal success with probe vs packer elements, it was decided to apply formation-pressure-while-drilling (FPWD) technology. The key questions with FPWD in this environment are: Can we achieve a good transient profile and what is potential impact of supercharging? These questions were addressed with advanced prejob modeling, which enabled determination of an optimized pretest configuration and testing procedure to minimize potential supercharging effects. While drilling, stage-in procedures were used, and mud logging total gas data were gathered to identify areas of liberated gas. Pre-run wireline petrophysical data were gathered to characterize the Petrophysic of the reservoir and to calculate an intrinsic permeability profile. Ultrasonic borehole images and caliper data were used to determine the principal horizontal stress directions, fracture frequency, and orientation and to confirm the stratigraphyc dipping of the structure. Combined, this information allowed a focused orientation of the FPWD probe and optimal station selection avoiding fractures and breakouts. This novel approach resulted in 100% seal success, >50% improvement. Four days of rig time were saved, and the required data were obtained.