ABSTRACT:

Understanding compaction behavior is essential for reliable pore pressure prediction, reservoir characterization, and safe drilling operations in sedimentary basins. The study aimed at evaluating the effect of compaction using petrophysical properties across eight(8) Wells and an accurate prediction of the sub-surface pore pressure, a basic requirement for safety, economics and efficiency in exploratory drilling and production of oil and gas. The capacity of assessing both short- and long-term rock behaviors according to the interaction between distinct parameters of rock texture, petrophysical and mechanical properties are highly instrumental to several geoengineering materials (Askaripour et al., 2022). This study presents a compaction trend modeling approach using well-log data from selected reservoir wells ‘Z’ field, Central Depobelt of the Niger Delta. Shale intervals were identified and normal compaction trends (NCT) across the study wells were established. The petrophysical characterization estimated across Wells A – H are shown in Table 1, while the compaction trend delineation obtained from petrophysical parameters for Sonic transit time (Dt), Compressional velocity (Vp), Density(ρ), Porosity (ϕ), and Overburden stress (σstress) for wells A – D and E – H are shown in Figs. 3 – 4, 5 – 6, 7 – 8, 9 – 10, and 11 – 12, respectively. From the estimated result for Dt across Wells (A – H), Well A had the highest average value of 117.93 us/ft, while Well H had the lowest average value of 87.64us/ft. For Vp, Well H has the highest average value of 3549.58m/s while Well A had the lowest average value of 264.82m/s. For Density(ρ) and Porosity(ϕ), it was observed to have the highest average value of 2.52gcc and fractional value 0.38 in Well H and A, with lowest average values of 2.21gcc and fractional 0.20 in Well A and H, respectively. Overburden stress(σ), was estimated to have the highest average value of 1.09psi/ft in Well H, with the lowest average value of 0.96psi/ft in Well A. Figs. 3 and 4 indicates that the Dt decreases with depth as compaction of rock increases with increase in ρ and Vp. The increase in Dt directly corresponded to higher ϕ values (slower travel). The Vp against depth profile across Wells A-D and E-H shows that Vp increases with depth, leading to higher compaction, because as sediments are compacted, porosity was observed to decreases across all Wells, hence grains are more tightly packed, and the rock matrix becomes stiffer which led to consolidation. This implies that as formation rocks become more compacted (denser) and less porous, their compressional wave velocity increases with decrease in sonic interval travel time, except where high porous and less dense zones were experienced that could initiate the reverse case of the trend, which possibly suggests the presence of abnormal pressure as observed from other complimentary profiles within the study area.

Keywords: Compaction trend, Petrophysical characterization, Overpressure, undercompaction, well logs data, Central Depobelt, Niger Delta.