Stress-Dependent Petrophysical Properties of High Heterogeneous Carbonate Tight Oil Reservoirs

Abstract

Abstract As a reservoir is depleted due to production, pore pressure decreases leading to increased effective stress which causes a reduction in permeability, porosity, and possible pore collapse or compaction. Permeability is a key factor in tight reservoir development; therefore, understanding the loss of permeability in these reservoirs due to depletion is vital for effective reservoir management. The paper presents a case history on a tight carbonate reservoir in Iraq which demonstrates the behavior of rock permeability and porosity as a function of increasing effective stress simulating a depleting mode over given production time. The experimental results show unique models for the decline of permeability and porosity as function effective stress. This stress sensitivity is even more pronounced in cores with lower initial porosity and permeability. The pores’ size and shape, as well as mineral content provide important reasons for various functions in the stress-dependent behavior of the porous media. Additionally, mercury injection capillary pressure (MICP), thin section (TS) results, scanning electron microscope (SEM) data, and X-Ray diffraction (XRD) are incorporated to relate the microscopic controlling factor to stress sensitivity behavior of this reservoir formation. The results indicate that permeability is more sensitive to effective stress than the porosity. Different responses to the stress of similar initial permeability are discussed according to their mean hydraulic radius (MHR). Distinguished fabric signatures for the studied reservoir is identified from grouping the MHR-permeability relations, which is significant and can provide insight on the heterogeneity of a given reservoir and how it is related to pore size distribution. This grouping mode provided better data allocation than depending on other parameters such as; conventional samples’ initial petrophysical properties, pore size distribution, MHR values, reservoir stratigraphic units division, and clay content for the samples, which all failed in achieving reasonable data grouping for the tight reservoir under study. A relation of the viscous behavior (ductility/brittleness) of this tight reservoir to permeability and stress is examined.