A Model of Pore Pressure and Effective Stress Changes During Hydrocarbon Depletion by Slowness Integrated Data Analysis
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This paper focus on the use of slowness data analysis to identify and characterize the pore pressure and effective stress changes during oil field production, since the pore pressure estimation plays an important role in oil and gas industry. Pore pressure estimation is included in substantial prerequisite before taking a major step to start drilling, production or recovering a well. Moreover, the precise pore pressure estimation will determine the safety, profits and success in both production and recovery. To accomplish the objective of this study, the methodology consists of several broad stages. It dealt with field mapping to create stratigraphy model, coring/sampling, velocity measurement with saturations and pressures (i.e. pore and confining) variations, velocity analysis for constructing the empirical equation of pressure, saturation and pore pressure relationship in each facies, pore pressure and effective stress changes estimation model from interval velocity and transit time analysis. The pore pressure and effective stress changes model was established for characterizing the changes in pore pressure and effective stress due to hydrocarbon production. Variation in fluid saturation was considered to hydrocarbon production scenarios. During the reservoir production, the fluid flows out leading to a reduction in pore pressure. This reduction produced slowing seismic velocities and inversely to effective stress. The characterization of pore pressure and effective stress changes due to changes in fluids saturation linked to velocity anomalies can be useful in monitoring hydrocarbon production.
KeywordsPore pressure Effective stress Hydrocarbon production Overpressure mechanism Velocity
This study focus on the characteristics of pore pressure and effective stress changes during hydrocarbon depletion by integrating the slowness data analysis as the changes of factor from variances reservoir properties. Samples from outcrop inside the Miri formation were used in this analysis for representing the hydrocarbon reservoir [1, 2, 3, 4, 5, 6].
Based on overpressure mechanism analysis in West Baram Delta, two overpressure mechanisms were characterized. Disequilibrium compaction or undercompaction could be identified in all wells whilst fluid expansion mechanism could not be determined in onshore area and area near proximal delta. According to this condition, this study was limited to pore pressure changes due to hydrocarbon production in disequilibrium compaction mechanism [7, 5, 8].
Characterize and investigate the pore pressure and effective stress changes through laboratory analysis with respect to the variance of overburden pressure, fluid saturation and temperature. Samples for variance porosity and permeability was used in laboratory analysis as sand reservoir representation and variance fluids saturation as hydrocarbon production representation was the main objective in this study.
The model was developed using the response of velocity from variance of parameters including fluid saturation, confining pressure and pore pressure temperature, where the temperature data came from well data analysis. In normal pressure and disequilibrium compaction condition, temperature data were collected and plotted against confining pressure for determining the linear trend. The relationships between pressure and temperature were used in the laboratory analysis. The effective stress was determined using velocity analysis modified by Eaton equation and pore pressure was determined by direct calculation; (Overburden pressures minus effective stress) .
3 Pore Pressure, Effective Stress and Temperature Characterization and Models
This was an enhanced model from the previous one as it elaborated all the parameters to characterize the pore pressure and effective stress changes due to hydrocarbon production. Temperature and P-wave velocities response were applied to develop the model. Based on the model, the conclusion was determined, which means that during the compaction, the overburden pressure increased due to compaction and led temperature and effective stress to increase. This condition developed the high P-wave velocities. Another condition, due to hydrocarbon production the pore pressure was depleted due to loss of fluids inside the formation. The depletion of pore pressure tend stress to increase to cause the effective stress to increase and produce high P-wave velocities.
Generally, the model was well-determined and established for characterizing the pore pressure and effective stress changes due to hydrocarbon production. In basic definition, the pore pressure was affected by hydrocarbon production. Variance of fluid saturation defined the hydrocarbon production scenarios. During the production, the fluid inside the formation pushes out leading to make the reduction of the pore pressure. This reduction produced low velocities and contradicted the effective stress. Unfortunately, this model was limited to disequilibrium compaction only, because of the lack of direct temperature data and the difficulty to map the temperature behavior in fluid expansion mechanism.
Consider the models the pore pressure changes can be analyzed and predicted for development of oil field purposes. Considering the changes of pore pressure, the accuracy of pore pressure prediction is very influential to safety and economic drilling with respect to the reservoir condition (normal pressure or overpressure zone). Determining overpressure zone is necessary for the petroleum industry because drilling into overpressure zone can be very risky and hazardous, hence fluids from this zone can escape rapidly. Specifically, an accurate determination of pore pressure has useful purposes for assisting drilling engineers in planning mud program and casing design in anticipated high overpressure zone and preventing a variety of drilling hazards, i.e. wellbore collapse, loss of circulation, stuck pipe and other possibilities.
In addition, the characterization of pore pressure and effective stress changes are due to the close relationship between hydrocarbon production and velocities anomalies that can be used in rock physic studies. The rock physics is the bridge between geophysics and geology. In fact, this area tried to quantify the seismic values based on its reservoir properties. Additionally, this model can be used for predicting 4D seismic after production and AVO analysis for a better hydrocarbon prediction. Since this model provides the variances of velocities response due to saturation, pore pressure, overburden pressure and temperature, it can be used to predict the next target and distinguish the reservoir variation due to its complexity
The authors express their gratitude to Universiti Teknologi Petronas (UTP) for providing laboratory facilities and financial assistance to this study.
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