This paper presents an innovative log-based method of determining pore volume compressibility as a function of pore pressure depletion. The approach considers changes in reservoir stress associated with pore pressure change (stress path) and incorporates constraints that ensure deformations are within elastic bounds. The approach incorporates the effect of stress anisotropy by using elastic moduli derived from stress-strain curves under simulated triaxial loading conditions. The triaxial condition pore volume compressibility was then converted to that of unaxial strain equivalent, which best describes the existing reservoir characteristics. The proposed methodology is particularly useful for predicting pressure-dependent pore volume compressibility where core specimens are either not available or in situations where laboratory measurements are prohibitively laborious and time consuming. For input, the method requires bulk modulus, compressive strength and other mechanical properties that characterize an elastic material, preferably predicted using a log-based mechanical property algorithm in order to generate a foot-by-foot profile of pore volume compressibility. A continuous profile of uniaxial strain pore volume compressibility with depth from log provides quick assessments of pore volume compressibility variations across the reservoir intervals. It is also useful and cost-effective for constraining pore volume compressibility of all the reservoirs penetrated by the well (and logged) but with only limited core data available for calibrations. The example shallow oil well data illustrates that pore volume compressibility decreases with decreasing pore pressure (or increase effective stress). An inverse pore volume compressibility to strength relationship was also observed. It was also observed that pore volume compressibility decreases with increasing porosity until the effective porosity reaches a critical minimum value. At porosity higher than the critical value, the pore volume compressibility increases with increasing porosity. This may suggest that reservoirs with a porosity less than the critical value are more likely to be under pressure drive, while reservoirs with porosity higher than the critical value are more likely to be under compaction drive. Introduction Pore volume compressibility, defined as the relative change in pore volume of a rock with respect to a change in pore pressure, is of fundamental importance in reservoir evaluation and management. It is an important parameter in material balance calculations and water/compaction drive performance studies. Its importance is becoming even more critical with recent fervent deepwater exploration and exploitation activities. These reservoirs, due to their depositional environments, tend to be weakly consolidated and oftentimes over-pressured. Compaction as a result of fluid withdrawal has major implications on reserve estimation, reservoir performance, casing integrity and seafloor subsidence. Due to the high risks and high level of uncertainties involved in deepwater projects, accurate estimations of pore volume compressibility, therefore, play a vital role in the project economics. In conventional depletion type reservoirs without strong pressure supports, reducing the fluid pressure causes changes in the reservoir effective stresses, which subsequently impact the volumetric changes of pore spaces. The engineering parameter quantifying these volumetric variations is pore volume compressibility.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPore Volume Compressibility (PVC) is one of the most important parameters for proceeding with project sanctioning. An accurate estimation of pore volume compressibility of reservoir rocks is essential for compaction evaluation, reservoir drive determination, reserves estimates, reservoir pressure maintenance, casing collapse analyses, and production forecasting. This information is then used in modeling the reservoir and calculating the economic value of the project. Thus, obtaining credible indications of this value early in the well evaluation process is invaluable. Unfortunately, this data is usually required long before a conventional core can be obtained and mechanical rock properties measured. Furthermore, cores are only available at discrete points along the wellbore requiring the data to be extrapolated to the missing sections.To accommodate the need for this data early in the project and to circumvent the short comings of core-based information gaps in the wellbore, a cost effective approach that utilizes common wireline data is used to obtain a continuous profile of the pore volume compressibility with depth shortly after wireline logging operations have concluded. The method employs a log-based mechanical property program that simulates triaxial loading to obtain static elastic moduli as well as rock strength. The resulting rock mechanical properties are then converted into their uniaxial strain equivalents and used to determine the pore volume compressibility. The process is repeated for several drawdown stages with the condition that the deformations are contained within elastic limits. This yields the complete reservoir compaction trend as a function of reservoir depletion.The log-based model was recently validated by comparing with lab-derived results from an offset well in deepwater Gulf of Mexico. The results indicate that the uniaxial pore volume compressibility obtained from the log-based method matches well with the results obtained in the laboratory. This fact suggests that the log-based approach should be utilized with a high degree of confidence to determine the PVC in the absence of core data, insufficient depth coverage of the cores, and/or to validate the core results.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPore Volume Compressibility (PVC) is one of the most important parameters for proceeding with project sanctioning. An accurate estimation of pore volume compressibility of reservoir rocks is essential for compaction evaluation, reservoir drive determination, reserves estimates, reservoir pressure maintenance, casing collapse analyses, and production forecasting. This information is then used in modeling the reservoir and calculating the economic value of the project. Thus, obtaining credible indications of this value early in the well evaluation process is invaluable. Unfortunately, this data is usually required long before a conventional core can be obtained and mechanical rock properties measured. Furthermore, cores are only available at discrete points along the wellbore requiring the data to be extrapolated to the missing sections.To accommodate the need for this data early in the project and to circumvent the short comings of core-based information gaps in the wellbore, a cost effective approach that utilizes common wireline data is used to obtain a continuous profile of the pore volume compressibility with depth shortly after wireline logging operations have concluded. The method employs a log-based mechanical property program that simulates triaxial loading to obtain static elastic moduli as well as rock strength. The resulting rock mechanical properties are then converted into their uniaxial strain equivalents and used to determine the pore volume compressibility. The process is repeated for several drawdown stages with the condition that the deformations are contained within elastic limits. This yields the complete reservoir compaction trend as a function of reservoir depletion.The log-based model was recently validated by comparing with lab-derived results from an offset well in deepwater Gulf of Mexico. The results indicate that the uniaxial pore volume compressibility obtained from the log-based method matches well with the results obtained in the laboratory. This fact suggests that the log-based approach should be utilized with a high degree of confidence to determine the PVC in the absence of core data, insufficient depth coverage of the cores, and/or to validate the core results.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe scope of applied geomechanics in the petroleum industry has been on the rise over the past decade. Geomechanics analysis has shown to increase the overall value to various projects and create positive technical synergies between various groups involved with project development. This paper highlights a comprehensive geomechanics study carried out on a deepwater Gulf of Mexico (GoM) field for optimizing field development with respect to critical drilling, completion and reservoir issues. A detailed and well-calibrated wellbore stability model applied to the Medusa field has shown to reduce the drilling operational costs by enhanced well planning. The log-based geomechanics models used to evaluate sand production and pressure dependent pore volume compressibility have aided in reducing project risks and increasing project life. The intent of the paper is to show the practical risk reduction and cost savings that are possible through geomechanics analyses in high-cost, high-risk projects in deepwater arenas such as the Gulf of Mexico.

The scope of applied geomechanics in the petroleum industry has been on the rise over the past decade. Geomechanics analysis has shown to increase the overall value to various projects and create positive technical synergies between various groups involved with project development. This paper highlights a comprehensive geomechanics study carried out on a deepwater Gulf of Mexico (GoM) field for optimizing field development with respect to critical drilling, completion and reservoir issues. A detailed and well-calibrated wellbore stability model applied to the Medusa field has shown to reduce the drilling operational costs by enhanced well planning. The log-based geomechanics models used to evaluate sand production and pressure dependent pore volume compressibility have aided in reducing project risks and increasing project life. The intent of the paper is to show the practical risk reduction and cost savings that are possible through geomechanics analyses in high-cost, high- risk projects in deepwater arenas such as the Gulf of Mexico. Introduction Operations in deepwater are costly and time sensitive requiring optimum well construction design. Incorrect engineering analysis can affect the Net Present Value (NPV) of the project and also setback the future project objectives. In order to reduce operations and engineering uncertainties, a detailed geomechanics study was conducted for the Medusa field. The two main objectives of the study were:Cost saving during operationsRisk estimations, project planning and optimization. The deliverables for the geomechanics study were based on:Drilling issues comprised of wellbore stability analysis for the development well program.Completion issues relating to prediction of sand production and completion strategies.Reservoir management issues consisting of pore volume compressibility determination for various pay sands.Wider utilization of the study by mapping the mechanical characteristics of the reservoir, overburden and other relevant formations, to assist with frac and pack design and subsidence prediction. An extensive array of drilling, logging, core and reservoir data from the few existing exploratory wells in the field provided the necessary framework and calibrations to characterize in-situ stresses, formation mechanical behavior and mechanisms of potential borehole instability. The wellbore stability analysis, consisting of mud weight window prediction and optimal well trajectory analyses, was carried out for five of the development wells. The success of utilizing this stability analyses for two of the recently drilled wells are discussed, highlighting the value of conducting such studies on high impact wells. Sand production prediction was carried out for all the main sand packages identified and encountered by the existing wells in the field. Formation compressive strengths and other mechanical parameters were computed every 0.5ft across the entire pay zones using a proprietary log-based prediction/simulation program. Critical drawdown pressures to predict sanding potentials were derived based on the static mechanical properties and expected flow conditions. Such detailed depth-based sand production prediction aided the project engineers in determining optimal production as well as completion strategies.