Abstract

Since the original paper was presented at the 1994 International Conference on Horizontal Wells, numerous laboratory studies have been conducted using the procedures outlined. Statistics are presented from 37 dynamic leakoff and regain permeability studies performed on twenty carbonate and sandstone formations from Western Canada, with several drilling mud systems.

Despite the wide variation in cores tested, test conditions, and fluids used, distinct correlations are evident. Threshold inflow pressure and stabilized regain permeability depend strongly on the original reservoir quality and applied drawdown. Threshold inflow pressure has a well-defined inverse relationship with permeability, especially in carbonate cores. Stabilized regain permeability, which was previously shown to be a function of both volumetric throughput and applied drawdown, correlates strongly with the "Cleanup Pressure Ratio", CPR, which is defined as the applied drawdown divided by the threshold inflow pressure. The data analysis also identifies performance differences between drilling fluid systems.

The fact that stabilized regain permeability is strongly related to the cleanup pressure ratio suggests that the pressure ramping technique described in the original paper is a superior procedure to other recently published techniques which advocate the constant rate method to determine threshold inflow or "lift-off" pressure. The constant rate technique can be subject to error due to equipment limitations.

In the absence of laboratory data specific to a given reservoir, the correlations identified in this paper can be used during horizontal well planning and evaluation to estimate well productivity, inflow contribution profile, and drawdown required for effective damage removal.

Introduction

Due to the economic impact of poor productivity in horizontal wells, there has been significant efforts in recent years to improve laboratory test methods for assessing drilling induced formation damage. Traditional methods of performing laboratory coreflood experiments to quantify damage were simplistic in that little attention was paid to the effects of drawdown pressure and throughput on cleanup. Minimal effort was made to differentiate between mud cake damage and filtrate damage. Consequently, these methods did not always reflect field conditions, particularly, where due to heterogeneity, irregular cleanup occurred.

More comprehensive laboratory methods for evaluating drilling mud damage were introduced in 1994. Experimental procedures were presented which mimic the dynamic process of mud cake formation and filtrate loss during drilling. More importantly, the concept of threshold inflow pressure was introduced, which defined the minimum drawdown required to initiate inflow after mud cake formation. In addition, methodologies were presented whereby cleanup during backflushing of reservoir fluids was monitored, not only as a function of throughput, but also as a function of applied pressure drop or drawdown.

The methods presented in that paper demonstrated the variable nature of mud damage cleanup with respect to applied drawdown and with respect to reservoir quality. Both volumetric throughput and increasing pressure drop had a significant impact on regain permeabilities. All tests resulted in a finite measurable threshold pressure prior to inflow of reservoir fluids. For the two muds tested, threshold pressure had an inverse relationship with permeability. These variable cleanup characteristics were incorporated into a horizontal wellbore model and simulated for a heterogeneous reservoir, demonstrating how assumed productivities based solely on K-h profiles, without accounting for variable cleanup, resulted in optimistic predictions of critical coning rates.

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