Pressure-Dependent Pore Volume Compressibility – A Cost Effective Log-Based Approach

Abstract

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.

Utilization of Geomechanics for Medusa Field Development, Deepwater Gulf of Mexico

“…The compressibility drops in the first domain and then increases in the second domain with increase in porosity. Similar phenomenon has been observed by others and has been described in detail by Ong et al 18 . The first domain is the result of the reduced relative deformations due to increasing pore spaces (residing at the denominator in the expression for compressibility).…”

“…18 have presented a unique and innovative log based method of predicting pore volume compressibilities. This is especially useful in projects such as Medusa, where the reservoirs are weakly consolidated and either the cores are not available or the quality of cores could be suspect due to incorrect handling and plug preparation processes.…”

“…In order to obtain the appropriate C bc, care must be used to ensure that the rock sample is subjected to confining pressure that is in the range of the mean effective reservoir stress. The pore volume compressibility is then converted to the one under unaixial strain conditions-C pu, representing the true depletion conditions 18 . The conversion expression was proposed by Teeuw 20 as:…”

Underbalanced Drilling Borehole Stability Evaluation and Implementation in Depleted Reservoirs, San Joaquin Field, Eastern Venezuela

“…The rock mechanical properties of the formations were estimated from the log data using an in-house program 11 , which converts the acoustic (dynamic) log data into more representative static parameters. Using the program, Poisson's ratio, unconfined compressive strength (UCS), internal friction angle and Biot's constant were obtained for Merecure and San Juan formations.…”

Log-Based Pore Volume Compressibility Prediction-A Deepwater GoM Case Study

“…In this paper, the application of the log-based PVC calculation method proposed by Ong et al 1 is used to determine the PVC at several different stages of drawdown. The results are then compared to the laboratory derived PVC (obtained several months later) and the total reservoir compaction is computed.…”