Summary In hydrocarbon reservoirs, fluid types can often vary from dry gas to volatile oil in the same column. Because of varying and unknown invasion patterns and inexact clay-volume estimations, fluid-types differentiation on the basis of conventional logs is not always conclusive. A case study is presented by use of advanced nuclear-magnetic-resonance (NMR) techniques in conjunction with advanced downhole-fluid-analysis (DFA) measurements and focused sampling from wireline formation testers (WFTs) to accurately assess the hydrocarbon-type variations. The saturation-profiling data from an NMR diffusion-based tool provides fluid-typing information in a continuous depth log. This approach can be limited by invasion. On the other hand, formation testers allow taking in-situ measurements of the virgin fluids beyond the invaded zone, but at discrete depths only. Thus, the two measurements ideally complement each other. In this case study, NMR saturation profiling was acquired over a series of channelized reservoirs. There is a transition from a water zone to an oil zone, and then to a rich-gas reservoir, indicated by both the DFA and the NMR measurements. Above the rich gas, is a dry-gas interval that is conclusively in a separate compartment. Diffusion-based NMR identifies the fluid type in a series of thin reservoirs above this main section, in which no samples were taken. NMR and DFA both detect compositional gradients, invisible to conventional logs. The work presented in this paper demonstrates how the integration of measurements from various tools can lead to a better understanding of fluid types and distribution.
Multiple wellbore stability projects in the Western Desert of Egypt identified a strong sensitivity of the breakout gradient to wellbore inclination and a weak sensitivity to wellbore azimuth, from the Abu Roash to the Alam El Bueib formations. The breakdown gradient shows strong sensitivity to both wellbore inclination and wellbore azimuth. Tensile failures are a problem only in high deviation wells (deviation more than 55-60°) where the breakdown gradient will in general be lower. For highly deviated wells, the occurrence of tensile failure can be reduced by drilling close to the minimum horizontal stress direction.Breakout can be tolerated to some extent, so planning the drilling mud weight to completely eliminate it is not necessary. The extent to which breakout can be tolerated depends on the well deviation, as cuttings become more difficult to evacuate as the well deviation increases.As losses are very common in most of the Western Desert formations, the optimum drilling mud weight should be formulated as a balance between the risks caused by breakout and mud loss.Understanding the main causes of rock failure and applying the recommended failure mitigation measures resulted in successfully drilling four deviated wells in the area. IntroductionThe Western Desert of Egypt is a mature hydrocarbon province. Wells with increasing deviation are being drilled to maximize the hydrocarbon recovery. This leads, however, to a rise in the severity of the borehole instability events. The Western Desert is a tectonically active region where the drilling problems range from mud losses in natural fracture networks or fault zones to time-dependent instability and tight pulls or sticking events. Depleted reservoirs are also encountered, lowering the fracture gradient.Wellbore stability analysis is an important tool to anticipate drilling problems and mitigate failure. We use a systematic approach: first, we analyze drilling reports in offset wells to gather the evidence for borehole failure. Then we develop a geomechanical model and calibrate it until it accurately predicts the observed borehole failures. After model calibration, we can make borehole failure predictions for any wellbore trajectory, regardless of the well deviation or azimuth, and assess the instability risks associated with a particular trajectory. This analysis assures that we choose the optimum drilling mud weight.Applying this process to several wells in one of Gupco's fields in the Western Desert resulted in four successful new wells.
Women on the Frontline - An examination of work/life balance issues by women in the industry.
In Egypt's Western Desert, water saturation evaluation of the Upper Bahariya reservoirs is complicated by significant uncertainties in the resistivity-based saturation equation inputs, mainly the formation water salinity, but also the cementation factor and the clay cation exchange capacity (CEC).A novel dielectric multifrequency measurement was able to clearly identify zones in which oil was present, regardless of these inputs. It provided a residual oil saturation and clear differentiation of oil-bearing zones from water-saturated zones. The measurement also permitted us to apply constraints to the conventional saturation interpretation from shallow and deep resistivities. This process highlighted the variability of formation water salinity across the well and the mixing of filtrate and formation waters in the zone investigated by microresistivity tools.The dielectric tool also measures dielectric dispersion, from which a parameter (MN) related to the tortuosity of the conduction path can be extracted. This MN parameter was used to refine the water saturation computation from deepresistivity logs. Variation of the MN parameter between 1.8 and 2 was observed in the reservoir zones. In the zone of interest, a water saturation difference of 7-8 % was observed in the cleanest zone between water saturation computed using the variable MN parameter and that computed using a fixed cementation factor of 1.9. The water saturation recomputed using the data from the dielectric log matches well the irreducible water computed from the nuclear magnetic resonance (NMR) data.Dielectric dispersion was also used to derive the CEC, which was then input as a direct measure of shaliness in the deepresistivity saturation equation.The dielectric measurement increased the accuracy of the water saturation computation in this challenging environment and provided data that are not directly available with conventional logging.
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