The Impact of Structures and Tectonics on Pore Pressure Regime and Gas Reserves Of the Off–Shore Nile Delta of Egypt

Abstract

Pressure regime in the untested Nile Delta basin, a major concern of the exploration and drilling communities, is the possibility of encountering overpressure and water-bearing sands in the rising less section of the wells. Formation pressure, is the pressure of pore fluid, can be considered the major factor affecting drilling operations and well planning processes. If the pressure is not properly evaluated, it can lead to many drilling problems, such as loss circulation, blowouts, stuck pipe, borehole instability and excessive costs. In the well planning process, the knowledge of formation pressure is the foundation for many segments of the well design, such as casing programs, mud programs, cementing programs, tubing design and rig selection. Without proper prediction of formation pressure, the well plan may be inadequate. The elements of the work plan are proposed as follows: 1- Reviewing the general geologic setting of the Eastern Mediterranean region, with Particular emphasis on the off-Shore Nile Delta Provence 2- Nile Delta geopressure well analysis, which documents the results of pore pressure analysis in each well and defines the basic petrophysical trends. 3- Nile Delta Overburden Gradient is involved and results obtained from applying the best practices in the Nile Delta. 4- Fracture gradient in the Nile Delta and Fracture pressure are estimated. 5- Appendix summarizes the best practices for the methods and procedures used in determines the pore pressure and fracture gradient. The study documents the results of pore pressure analysis performed on 14 wells from the Nile Delta (water depths range from 19 to 2800 m). The study aims at developing best method(s) to estimate pore pressure regime. This study overall the results (geopressure trends, modeling parameterization summary, geological events and their impact on the pore pressure … etc.) Based on the geopressure analysis performed on the 14 wells, the following conclusions are obtained: 1. The pore pressure and fracture pressure can be computed, according to methodologies derived largely from the Gulf of Mexico basins and have been modified empreciding to suite the Nile Delta geological environment. 2. The calibrated sonic and resistivity models are important tools in the real time geopressure monitoring on the rig site during exploratory drilling. 3. The onset of over pressuring started in the Pliocene, at the beginning of the first major shale interval (Kafr El Sheikh) and increased rapidly just below the Messinian unconformity. Sand and shale ratio plays a dominant role in the position and development of over pressuring in the Pliocene section. 4. The presence of sealing at the top of Miocene provides the proper environment for accelerated the pressuring build-up, immediately below the seal as a result of the increased rate of fluid retention (under compaction) and to a minor contribution from the aqua thermal expansion and illitization. 5. When the structural data are available, it has been postulated that the structure of sand bodies affected the development of the over pressuring in these sands, when the well penetration is at the crest (or above the Centroid point) of these sands. 6. The pore pressure estimation model, based on the acoustic data, is uniquely defined for the Nile Delta, which applies well to the Pleistocene, Pliocene and in some cases to the Miocene/Oligocene sections. This model is based on the equivalent depth method. 7. Parameters for the sonic normal compaction trend are sensitive to the sedimentation rate in the Pliocene or younger sediments. This provides a rational sand for the sonic model calibration based on anticipated geological environment, if required for an exploration well application. 8. The Miocene and older sediments may be unloaded due to the high temperature, illitization, uplifting and erosion. Under these conditions, an unloading model is proposed for the pore pressure estimation. So, recommend Miller‟s method due to its simplicity. Both Bowers‟ and Eaton methods can be used with care. It is difficult to set generalized parameters for any model. 9. In the case of unloaded Miocene section, the traditional pore pressure estimation methods (Eaton, Equivalent Depth … etc.) will under estimate the pressure (for example in Tineh-1, the conventional method pore pressure gradient is 5ppg less than the actual). 10. Excluding of sedimentation loading, the most significant geological events which control the application of a geopressure models are the uplifting and erosion. Habbar-1, Aida- 1 and Tineh-1 wells are known to have significant erosion. Uplifting is known to be a factor in Tineh-1 but not in the others. 11. Erosion effect on the pore pressure development in the Miocene and older section, is a complex phenomenon, which can be attributed to many factors. The most notable factors are: grain cementation that may occur during sediments exposure to sea water or rain, amount of loading (burial) after the erosion took place and the possible pressure leakage during erosion. Tineh-1 well may be subjected to severe cementation after erosion more than the other wells (Aida-1 and Habbar-1). This may explain the large error in the predicted pore pressure from RFT measurements and mud weight. 12. Seismic velocity data are inherently of poor resolution and cannot be used reliably to predict pore pressure in the Miocene or older sediments. Since the pore pressure prediction study became fundamental in the pre-drill phase of challenging wells, the target is to consider the study as a routine job to be proposed to shareholders, both for the technical concurrence and for costs approval with the help of good velocity data, we can evaluate better the pore and we can calculate well the seal capacity and minimizing the well risk. So, we can use a new technology of broad band frequency to enhance the velocity content or use of AVA velocity or the 3D HDHR velocity picking. 13. Formulating the relation between Pore Pressure of the studied formations and Risk Evaluation, based on the Fore –Warning and Warning Stages of the Pore Pressure Regime and pore pressure affecting on the performance of reservoirs. 14. Utilizing the Pore Pressure Prediction as a future prospective tool in the new areas of exploration and future view for reserves oil and gas on the Off-Shore Nile Delta province of Egypt. Finally the main effect for accumulation pore pressure is tectonic elements which lead to occupied fluids and overburden sediments so the area which have heavy tectonic means pore pressure prediction is very important before drilling and pore pressure one of evidence of accumulation hydrocarbon.