Instrumentation Requirements for Kick Detection in Deep Water

Abstract

In deepwater drilling, increased emphasis must be placed on early kick detection. Using a recently developed computer program to model kicking wells, various types of kick detection instruments were evaluated to determine which are most useful and what sensitivities are desirable. Kick detection based on return-mud flow rate was found most beneficial. Introduction In offshore drilling operations, increasing water depth reduces the difference between the mud weight required to balance formation pore pressures and that weight causing formation fracture. Fig. 1 illustrates this point at the seat of a 3,500-ft (1070-m) surface casing string. The formation pore pressure gradient is assumed to be that of seawater, equivalent to an 8.5-lbm/gal (1020-kg/m3) mud weight. The casing-seat formation fracture gradient at zero water depth (i.e., on land) is assumed to be equivalent to a mud weight of 13.5 lbm/gal (1620 kg/m3) . Translating the same geological conditions to an offshore environment results in the situation shown in Fig. 1. The pore pressure gradient remains constant since the added overburden is seawater; however, the casing-seat fracture gradient decreases dramatically with water depth. If a 0.5-lbm/gal (60-kg/m3) margin is maintained relative to both the fracture and pore pressure gradients, the range of allowable mud weights is reduced substantially as water depth increases.Another way of illustrating the effect of increasing water depth is to consider the "critical kick size." This is the maximum gas influx volume that can be contained in a shut-in well without fracturing the rock at the casing seat. Influxes larger than this will cause an underground blowout if shut in. Fig. 2 shows the critical kick size as a function of water depth for kicks taken while drilling a 12 1/4-in. (311-mm) hole 5,000 ft (1520 m) below a 3,500-ft (1070-m) surface casing string. The drillstring consists of 5-in. (127-mm) drillpipe with 600 ft (180 m) of 8-in. (203-mm) drill collars. The gas is assumed to be a single bubble at the bottom of the hole. Mud weights of 9.5 and 10 lbm/gal (1140 and 1200 kg/m3) and underbalance conditions of 0.25 and 0.5 Ibm/gal (30 and 60 kg/m3) are considered. While kicks of 150 to 250 bbl (24 to 40 m3) are necessary to produce casing-seat failure on land, in 5,000 ft (1520 m) water, the critical kick size is reduced significantly.While there are numerous reasons for limiting kick size on land or in shallow water, this analysis shows that there is increased incentive in deepwater drilling operations to detect and control kicks at an early stage. The accuracy, sensitivity, and reliability of the drilling instrumentation are key elements for determining at what stage a kick will be detected and controlled by the drilling personnel. We have investigated the time-dependent evolution of kicks under a variety of assumed conditions to define better what instruments are useful for early kick detection and what levels of accuracy and sensitivity are desirable. Here, we used a recently developed computer program that models the transient behavior of a kicking well. Gaskick Program The Gaskick computer program was developed to simulate the evolution of gas kicks in a wellbore. JPT P. 1029

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