Formation fluids are displaced by drilling mud filtrate as a result of pressure overbalance during drilling. This process changes the petrophysical properties of the near-wellbore zone and creates an invasion zone that has a complex radial profile characterized by the partially decreased porosity, permeability, and altered saturations. Further, at the completion stage the well is cased, cemented, and then perforated to re-establish connection between wellbore and reservoir. During perforation, a shaped charge produces a jet of dense material traveling at very high velocity which penetrates casing, cement, and formation. The resulting tunnel is a rugose tapered cylinder roughly characterized by its diameter and total depth of penetration.One of the main goals of perforated completion is to ensure fluid flow from the productive reservoir interval to the wellbore. Equally important is the ability of the jet to penetrate beyond the zone of formation damage caused by drilling, connecting the wellbore to the virgin reservoir and alleviating the effect of formation damage on production. The ability to predict the invasion depth and the depth of penetration of downhole perforators is therefore critical for pre-job completion modeling.This work presents the results of numerical modeling predictions of both drilling mud filtrate invasion during drilling and jet penetration in rock during perforation. The invasion model is further applied to the well data interpretation, and a good agreement with log resistivity profile is shown. In addition, we review and discuss various empirical methods currently used in the industry to predict penetration depth. Despite a variety of available methods and published experimental data, penetration depth results are often inconsistent with each other and are of questionable use in predicting actual downhole penetration.We highlight the importance of combining accurate invasion and penetration models for the successful pre-job completion planning. The results should be used further with the well-inflow model to maximize well productivity and minimize the effect of formation damage.
In order to ensure well stability, distinguish high- and low- pressure zones and estimate the level of pressure depletion, information about formation pressure is necessary. Due to formation damage during drilling and mud filtrate invasion, true formation pressure cannot be measured directly when formation permeability is relatively small. Therefore, an accurate model of invasion profile is required to calculate true formation pressure from formation testing data. This is possible to achieve by combining drilling with LWD and/or wireline logging data. This paper describes a method of computing depth of invasion by inversion of resistivity logging data. We use resistivity image data to calculate flushed zone resistivity and induction logging data to compute true formation resistivity. This, in turn, provides an invasion zone profile and significantly reduces the ambiguity of possible solutions. Drilling regime, rate of penetration, wellhead pressure, and mud properties are used to calculate wellbore pressure. The changes in formation pressure during drilling are computed by the hydrodynamic model of invasion. We present the result of formation testing data processing for water-saturated reservoir. The true formation pressure is estimated using the results of inversion, namely, estimate of mud filtrate volume penetrated into formation. Drilling Mud Invasion During drilling, the pressure overbalance is created to provide well stability and prevent blow-outs. Due to this pressure difference a certain volume of drilling mud filtrate invades the formation. The composition of filtrate is different from that of the formation fluid. Depending on filtration resistance and time of action the depth of invaded zone can vary from a few centimeters to 0.8–1.0 m. Thus, logging measurements with relatively shallow depth of investigation (less than invasion depth) provide information about the disturbed part of the formation and not about the virgin formation parameters. In order to improve interpretation of logging data, it is important to be able to estimate rate of change of formation parameters during drilling. In this paper we present a method to estimate total volume of mud filtrate penetrating into formation. The results are used to improve interpretation of formation testing data. A method to evaluate mud invasion characteristics from resistivity logging data was suggested in Kashevarov et al. (2003). One of the ideas of the method is the change in salt concentration profile due to invasion. It is caused by different salinities of mud and formation fluids and changes in salt concentration and saturation profiles during invasion. This leads to the changes in the resistivity profile. Information about the latter is obtained using resistivity logging data acquired by tools with different depths of investigation. In this paper we used induction and micro-resistivity logging data to calculate the resistivity profile in the near-wellbore zone. The micro-resistivity tool has a very shallow depth of investigation and is therefore sensitive to the flushed zone, which is the nearest to the borehole. Induction logging has much larger depth of investigation and is used mainly to measure true formation resistivity. We use data acquired by both tools simultaneously to define general geoelectric model of the near-wellbore zone. By integrating the two resistivity logging methods we obtain more reliable results and a more detailed resistivity profile.
The presented study demonstrates the effect of different technological activities of drilling through a reservoir interval on induction logging while drilling and near wellbore resistivity. The drilling operations include drilling with different penetration rates, shutdowns for drill-pipe addition, back reaming and so on. To estimate this effect, numerical modeling has been applied for simulation of mud filtrate invasion followed by electromagnetic modeling of induction tool signals. When modeling the invaded zone, the developed algorithm takes into account the drilling mode and mudcake formation. The computed near wellbore distributions of invasion zone parameters such as water saturation and salinity are then used to determine the resistivity distribution in the near wellbore area. The obtained geoelectric model of the well and the invaded zone makes it possible to compute the induction signals measured while drilling. It has been shown that neglecting features of the drilling operation may lead to improper interpretation of logging measurements.
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