Enhancing Pore Pressure Analysis Through Integrated Prediction Filter Error Analysis

Abstract

Pore pressure, the pressure exerted by fluid in the subsurface, is widely affects oil and gas operation from basin wide exploration to field development. The quantification of pore pressure highly relies on decompaction method such as Eaton’s or Bower’s equation. Those equations are derived from Terzaghi’s Law and require overburden and effective stress to ultimately calculate the pore pressure. Overburden stress can be easily calculated from bulk density log while the effective stress has to be estimated through rather indirect approach using normal compaction trend on the acoustic log. The trend itself has to be estimated through decrement of claystone’s acoustic property. Claystone intervals have to be identified using facies sensitive log such as gamma ray. However, gamma ray measurement could be disturbed by acquisition error and acoustic log could be influenced by abrupt geologic event such as unconformity. Gamma ray values could be normalized through clay volume calculation. However, discerning acquisition error in acoustic log that needs to be ignored and geological events that has to be considered for the normal compaction trend is more problematic and requires additional advanced analysis. Integrated prediction filter error analysis (INPEFA) is a technique that could be used to analyzed depositional sequence based on deviations of facies sensitive log such as gamma ray. Well data from East Natuna were used to conduct this study. Existence of unconformity and overpressure has been detected in this region. Several sampling rates are used to generate short, medium and long INPEFA log. The result shows that the INPEFA is well correlated with the regional geology thus it could be used to adjust normal compaction trend. This paper is meant to elaborate how INPEFA could be implemented to assist pore pressure calculation through depositional sequence analysis that is well aligned with regional geology and ultimately assisting the creation of normal compaction trend by filtering acquisition error and geologic unconformity.

AAPG Datapages/Search and Discovery Article #90324 © 2018 AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment, Perth, Australia, June 6-7, 2018