Safe drilling is seen as a top priority in the exploration and production (E&P) activities of the oil and gas sector. Thus, having any abnormal formation pressure will directly influence drilling operations. Accordingly, ideal pore pressure and fracture modeling is highly required for wells’ planning and execution.
Pore pressure is the pressure exerted by fluids on the pore space of the matrix. Based on its value in comparison with the hydrostatic pressure; the fluid’s pressure, at a given point of equilibrium, can be classified into normal, abnormal, and subnormal.
Hence, any changes that occur in the pore pressure will deeply impact E&P. This is because it is one of the critical parameters for designating the reliability and success of casing design. It is important to highlight that casing design is a vital step for any successful well design.
A sample paper tackling this issue is entitled Pore and Fracture Pressure Modeling Using Direct and Indirect Methods in Badri Field, Gulf of Suez, Egypt. The paper was published in 2019 on the Journal of African Earth Sciences. It was authored by A.E. Radwan, from the Exploration Department at the Gulf of Suez Petroleum Company; A.M. Abudeif; M.M. Attia; and M.A. Mohammed from Faculty of Science, Sohag University.
The main objective of the paper is to make new advances in the pore pressure and fracture models for the Badri field at the Gulf of Suez, to specify the geo-pressured zones in the subsurface sedimentary section, as well as to understand the pore pressure distribution in subsurface formations.
Badri field is geologically known as the Suez rift and is located at the Gulf of Suez. Hydrocarbon was discovered in the Miocene successions, where oil and natural gas resources were locked into Belayim and Kareem reservoir formations. The rocks of these formations were composed of sandstone comprising the seafloor of deeper formations in Zeit, South Gharib, Belayim, and Kareem formations.
According to the paper, Badri field is bounded by an E-dipping extensional fault at the eastern and western parts with a respective throw of approximately 1,500 feet (ft) at the top of Belayim formation, and around 250–800 ft separating Badri field from El Morgan field.
Materials and Methods
The study of the Badri field was conducted on four wells; X1, X2, X3, and X4. The study used all available data including well logs (density, resistivity, neutron, and sonic), drilling data, and reservoir data. Furthermore, it depended on the integration of geology, in addition to accurate reservoir data and drilling events to reach an ideal geo-pressured evaluation.
The technology implemented in this study was Schlumberger’s Techlog® software. This software helped in the calculation and interpretation of the data for output models. The study was classified into five stages:
At this stage, all required geological, drilling, and reservoir data are collected. These data include mud and electric logging data, directional surveys, temperature, mud weights, equivalent circulation density, drilling events, core data, structure, reservoir pressure data, well and production reports, etc. Collecting these data will help in pore pressure interpretation.
Quality Control and Data Summarization
This stage is very critical because all unusual data affects the following processes. During this stage, all collected geological, drilling and reservoir data will be re-checked to assure their suitability to the calculation stage.
Moreover, this step includes environmental corrections, which entails correcting logging data for several unwanted environmental effects using suitable mathematical relationships and Schlumberger’s charts. For this study, it comprises some preliminary corrections for borehole effects, formation temperature with depth, f Gamma-Ray for borehole variations, etc. These corrections are carried out before the interpretation processes.
This stage comprises several calculations including lithology discrimination, overburden stress, hydrostatic pressure, pore pressure, and fracture pressure.
For lithology discrimination, Gamma-Ray; density; sonic; and neutron logs were used to discriminate between sand and shale lithology. It is worth noting that the selection of shale intervals greatly affects the results of the pore pressure fracture gradient (PPFG) model.
For the overburden stress (σV), it is deﬁned as the weight of the overburden rock. It is computed by using the density log.
When it comes to estimating the hydrostatic pressure, water density value is used as the average value of water density which is used in the hydrostatic pressure gradient.
After selecting the shale intervals, the normal compaction trend (NCT) will be performed, noting that the interpreter experience plays a major role in picking NCT for each type of logs including resistivity, sonic and density logs.
This is followed by the estimation of pore pressure. Then, once the overburden and pore pressure are calculated, the fracture pressure can be estimated.
During this stage, drilling tests and events, and reservoir pressure are used to adjust the pore pressure and fracture gradient calculation. Furthermore, the interpretation of the output is used to evaluate the pressure system.
In this stage, all data are plotted to create the PPFG model for the area under study including formation tops, overburden stress, interpreted pore pressure, fracture pressure, drilling events, repeated formation test and leak off test, etc.
Results and Recommendations
This study managed to analyze several sets of data including well logs, mud logs, drilling reports, and reservoir data. Accordingly, the study resulted in estimating pore pressure of the four wells under study; X1, X2, X3, and X4. In addition to that, the pore pressure calculations estimated by indirect calculations were linked to direct measures to define the geo-pressured areas in the Badri field. It was found out that there were four over-pressured zones and two subnormal ones. On the other hand, there were some normal pore pressure zones observed in the upper part of Zeit formation, as well as three parts in Belayim formation.
Consequently, the study suggested using the high mud weight at the base of Zeit shale and the south Gharib formation to reduce problems faced during drilling. However, low mud weight is highly recommended to be used in the reservoir section to avoid any lost circulation.
Furthermore, the paper weighs in on the importance of implementing several techniques for pore pressure estimation in the Gulf of Suez. This will help understand the uncertainties of each method to determine the best one for studying the field basin.