Damage Ratio from With Variable Drill-Stem Tests Back Pressure

The drill-stem test [DST] is a temporary completion designed to sample the reservoir fluid and to establish the possibility of commercial production. It also yields values for the permeability capacity of the formation, for wellbore damage and for static reservoir pressure. The conventional interpretation is based on the assumption that the production rate is constant during the flow period. In fact, flow is not constant because the fluid column rising in the drill pipe exerts a time-variable back pressure on the face of the formation exposed in the wellbore. The effect of the resulting variable flow rate on damage ratio obtained from conventional DST analysis is studied in this paper. For the case of high fluid productivity, conventional analysis yields a higher formation capacity and a lower damage ratio than the true values. The error in the damage ratio is particularly large. The method of Odeh and Selig1 will improve the flow capacity calculation considerably. However, they do not calculate the formation damage. A method for calculating the damage ratio is suggested here. INTRODUCTION The drill-stem test [DST] is a production test carried out through a temporary completion; it is designed to sample the reservoir fluid and to determine the possibility of commercial production. The test is carried out by attaching a fluid sampler to the drill collar and lowering the sampler to the zone to be tested. One or more packers, which are part of the tool, are set to isolate the formation from the mud column in the wellbore; then the formation fluid is directed into the sampler and subsequently into the drill pipe. A clock-driven recording gauge records the pressure variations. After a short period of production, the valve between the drill string and the sampler is closed; the recording of the bottom-hole pressure continues and corresponds to a short buildup test. The plot of this pressure on a linear scale vs [T+ ?]/? on a logarithmic scale is a curve, an analysis of which yields values for formation capacity [kh], wellbore damage, and static reservoir pressure.2