Abstract
Pore volume compressibility is a critical parameter to evaluate the reservoir potential and recovery factor. It is ideally determined through uniaxial core measurements, but may also be inferred through conversion of hydrostatic core analysis to ‘effective’ uniaxial values. Alternatively, determination of pore volume compressibility (PVC) through logs offers the option to devise an early PVC strategy for field development without extensive SCAL. While some successful siliciclastic case studies are available for log-based PVC estimation, little is established for similar techniques in carbonates due to its complex pore network. Devising a truly log based method that can serve equally well in carbonates is a challenge to the industry. This paper describes a new technique for log-based estimation of PVC.
The evaluation of dry frame bulk compressibility and matrix bulk compressibility are the basic pre-requisites for PVC determination in a rock poro-elastic model. Volumetric log analysis is utilized to determine relative components of complex matrix volumes and porosity. Hashin-Shtikman (HS) bounds are adopted to determine matrix elastic properties of a composite system. To determine the dry frame elastic properties, the present technique utilizes a Differential Effective Medium model and adopts the concept of effective aspect ratio that simulates the rock elastic behavior. It employs the assumption of constant rock shear compressibility under wet and dry conditions to predict dry rock behavior. In an iterative procedure, the approach predicts effective aspect ratio, dry frame bulk compressibility and in-situ pore volume compressibility for the each log depth. The shear compressibility input for DEM analysis is derived through sonic and density logs.
Thes technique is implemented in dolomite dominant complex carbonates in the south of the Sultanate of Oman. The model is validated in test wells where excellent match is observed between log-computed PVC and uniaxial core values. It is observed that pore volume compressibility is strongly dependent upon the effective aspect ratio and the porosity. This study delineates this relationship. For comparison, an inverse Gassmann's equation was used to determine dry bulk compressibility and found to highly underestimate pore compressibility.
