A Case Study Integrating the Physics of Mud-Filtrate Invasion With the Physics of Resistivity Logging




George, Bovan Kooruvelil

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The focus of this thesis is an active gas-producing field. Lithology consists of inter-layered carbonates along with fine-grained clastics and shales. Recent wells have been drilled with water-base mud of salinities close to 2,000 ppm. On the other hand, the salinity of the connate water is very high, of the order of 200,000 ppm. Two major challenges faced in the petrophysical evaluation of these reservoirs are: (1) deep mud filtrate invasion beyond the depth of investigation of the induction logging tools, and (2) low deep resistivity readings which cause biased estimates of in-place gas reserves. The low deep resistivity readings have been attributed to the presence of a low resistivity annulus around the borehole. However, the origin and extent of the annulus have never been explained from basic petrophysical principles. A consistent explanation of this phenomenon is presented in this thesis based on the physics of mud-filtrate invasion. A 30 ft-thick carbonate formation in a key well was selected as a representative zone. Gas saturation of this formation is about 80-85%. The process of mud-filtrate invasion was modeled with a two-dimensional chemical flood simulator that included the effect of salt mixing between mud filtrate and connate water. Extensive core and well-log data were used to constrain petrophysical parameters such as porosity, absolute permeability, relative permeability, and capillary pressure during the simulations. In addition, samples of mud and mud filtrate provided adequate information to constrain fundamental parameters governing the process of invasion such as mud cake growth and effective flow rate across the mud cake. Radial resistivity profiles were obtained from the simulated profiles of water saturation and salt concentration using Archie’s law. These profiles clearly showed the presence of a low-resistivity annulus in the transition region between the flushed and virgin zones. Numerical simulation of induction logs was performed to validate the agreement between the mud-filtrate invasion model and the available wireline induction logs. These log simulations provided a way to further constrain the parameters governing the process of mud-filtrate invasion, especially time of invasion. An
8extensive sensitivity analysis was performed to quantify the effect of additional petrophysical parameters, such as capillary pressure and relative permeability, on the radial profiles of water saturation and salt concentration. Results from this work show that standard induction tools are extremely sensitive to a low-resistivity annulus, which effectively reduces their depth of penetration and hence cannot accurately determine gas saturation in the virgin zone. It was also found that the pre-annulus and annulus segments of the resistivity profile remained insensitive to initial water saturation, thereby impeding an indirect estimation of in-situ gas saturation. By contrast, laterolog measurements remain only slightly affected by the presence of a low-resistivity annulus.The sensitivity analysis described in this thesis provides a rigorous quantitative method to assess chemical effects of different types of mud on the invaded zone prior to drilling. It also provides a means to design and time borehole logging operations and interpretation procedures that can yield accurate estimates of in-situ petrophysical variables in the presence of deep invasion.


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