Formation and Fracture-System Compressibility Tracked Through Interference Testing

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225023, “Tracking the Formation Compressibility in Carbonates and Determination of Fracture-System Compressibility Through Interference Testing: Case Studies in Naturally Fractured Reservoirs From Mexico,” by Ricardo Alcantara, SPE, Pemex and University of Aberdeen; Jose L. Bashbush, SPE, UNAM; and Gorgonio Fuentes-Cruz, SPE, IMP, et al. The paper has not been peer-reviewed. _ Carbonate formations represent a challenge for reservoir engineering given their high heterogeneity as a result of their sedimentary origin and post-deposition diagenetic effects. Because the determination of the rock-compressibility value in carbonate naturally fractured reservoirs (NFRs) is paramount, a need arises to generate rock-compressibility correlations for the NFRs of Mexico. The developed methodology consisted of compiling all information from laboratory analysis of cores to generate a database. With this information, it was possible to correlate the compressibilities obtained and discretize the compressibility of the matrix and fractures at reservoir scale. Rock Compressibility and Effective Stress Influence in Carbonate Cores In hydrocarbon-producing reservoirs, the formation is subject to external (overburden pressure) and internal (pore pressure) stresses, which interact by altering the balance within the medium when the extraction of hydrocarbons begins. In hydrocarbon reservoirs, compressibility influences the efficiency of hydrocarbon recovery directly. Therefore, understanding the compressibility of the rock is essential in evaluating the response of the reservoir to fluid extraction and optimizing exploitation strategies. The definition of compressibility of a material (solid, liquid, or gas) at a given temperature is the change in volume per initial unit volume caused by a pressure variation. The complete paper provides a series of equations related to the determination of compressibility. Obtaining rock-compressibility (and, hence, pore effective compressibility) measurements and simulating field stress and pressure changes can be challenging. Studying mechanical influence in certain rocks can be difficult considering the very long time required for sample depletion; in other cases, it is hard to simulate changes in stresses close enough to field conditions. Moreover, sample size may not represent the rock mechanical behavior. One of the more important aspects to consider is the fact that, when samples or cores are detached from their in-situ environments, they are disturbed because of coring and stress relaxation, mud contamination, weathering of the rock, handling and transportation, and formation-fluid expansion; thus, laboratory measurements must perform certain corrections considering these issues. Different rock-sample types for laboratory measurements exist, such as plugs obtained from conventional cores, full-size cores, percussion sidewall cores, and drill cuttings. Likewise, distinct categories of laboratory tests have developed, such as compressive and tensile tests (based on loading pattern), static and dynamic tests (based on loading type), destructive and nondestructive tests (based on the effect on samples), and conventional and special tests (based on laboratory routines). In summary, each type of test has its own applications and limitations.