An experimental study on variation of pore pressure and wave velocity in carbonate rocks: impact of depositional facies and diagenesis

Over the years, the observation of wave velocity in relation to pore pressure variation has emerged as an intriguing topic in reservoir characterization. Considering the significant role of geological deposition processes (e.g., diagenesis and facies) on reservoir pressure, a different perspective is offered on the influential factors affecting wave velocity response to pore pressure changes. In this context, core samples from different depositional environments in the carbonate reservoir were used to examine how P- and S-wave velocities vary with confining pressure and pore pressures. These variations were measured through specific triaxial testing simulating hydrocarbon reservoir conditions. The selected test was implemented to directly investigate the interactions of the rock’s microstructure and pore fluid pressure. Next, the influential parameters of the sensitivity of wave velocities were analyzed according to physical, mechanical, and sedimentary texture. The results showed that porosity and compressibility are among the effective parameters on pore pressure variation. Furthermore, there are other determinative factors in the sensitivity of wave velocity regarding the pore pressure variation, which can be controlled by rock microstructure. As novel findings, the impact of geological diagenetic features (including micrite cement contribution, neomorphism, fracturing, dolomitization, and dissolution) was quantified to examine the variation in pore pressure and wave velocity. The results indicated that these factors had the most significant influence on pore pressure detectability. Furthermore, the results showed that highly compressible porous carbonates with packstone texture were more sensitive to pore pressure, compared to tight carbonates with highly rigid frameworks, which are found in mudstone texture. Also, observations showed that confining pressure is more effective than pore pressure in combination with each other to obtain the physical properties of a reservoir rock. The findings of this research offer valuable insights for predicting pore pressure in carbonate reservoirs for exploration and development programs.