The normal compaction trend and the pore pressure gradient play a very important role in oil wells design for drilling operations. It is essential understand the physical principles originating these pressures and evaluate the models of quantification for a particular gepgraphycal area. In this paper, we show the results from our analysis of geopressures in 11 (eleven) offshore wells drilled in the Offshore Area in Gulf of Mexico from PEMEX. Our work focuses in the normal compaction trends using curves of Resistivity Logs. Moreover, the Eaton´s model was regionalized in order to defining the magnitude of the pore pressure with high precision. From those results we inferred that the accuracy of the predictions depends of slope from normal compactions trend. Finally, using the profiles of real pressure data, we built a ‘Pore Pressure Cube’ for the area of study, which simplifies the quantifications of pressures during the process of planning and design of the wells.
There are several mechanisms originating abnormal pressures. This phenomenon is related to physical, geological, geochemist and mechanical processes. Frequently is difficult to establish what phenomena is the most important for a particular geological frame due to their dependences in the abnormal pressures origins. Between the causes of abnormal ressures, referred in the literature1, we picked up the following:
Compaction disequilibrium
Tectonic stress
Aquathermal expansion
Mineral transformation
Hydrocarbon Generation
Thermodynamic effects
Osmosis
Hydraulic head
From our point of view the main cause cited of abnormal pressure behaviour is sediment compaction disequilibrium. Present work focuses in the identification of compaction changes. Our approach considers Terzaghi2 theory, and Hottman & Johnson3 Log Analysis method in order to define changes in the shale compaction trend and we exploit Eaton's4 Pore Pressure Equation for resistivity logs. We describe pore pressure results using Eaton equation considering resistivity compaction trends from Mexican offshore wells. Further, we analyze resistivity compaction trends and defined a series of trends that we call ‘overlay graph’ of resistivity trends.
Finally, we define a new alpha exponent for Eaton's equation; such changes in the exponent allow us to define pore pressure behaviour with more accuracy, this fact was successfully validated using resistivity logs from several offshore wells.
From previous works in pore pressure prediction to offshore Mexican wells5,6. We found that original Eaton equation for resistivity and transit time ‘overestimate’ pore pressure than results obtained from well real measures.
In figure 1, the red line at right, show pore pressure with 1.2 Eaton exponent; green lines; in same figure, show ECD and mud weight used to drill well. We can see that pore pressure is higher than real mud weight.
From this analysis we already verified that when we used original Eaton exponents, predicted pore pressure is higher than real. We considered that those coefficients in the Eaton equation must be adjusted to obtain more accurate results for Mexican Offshore wells.
