Abstract

This paper addresses kicks that occurred in deep, high bottom hole temperature gas wells while tripping in the hole. These kicks were initially thought to be caused by a combination of swabbing while tripping out of the hole, gas entering the wellbore while coring, and core gas bleeding while tripping the core out of the hole.

An alternative explanation for these kicks is presented here. An underbalance condition created while tripping in the hole is proposed. The pressure transients while running in the hole are modeled and it is shown that a combination of reduced hydrostatic pressure due to high temperature and a secondary swab effect while tripping in, can lead to periods of underbalance where the fluid hydrostatic head is less than the formation pore pressure. This could be an important contributing factor leading to the kick condition observed in these wells. The paper describes the modeling, the influence of various parameters on the pressure transients while tripping in the hole, and presents results and sensitivity analyses.

Introduction

When a tubular (such as a drillstring or casing) is moved up or down in a hole filled with a fluid, transient pressure fluctuations occur, causing the fluid pressure at a given depth to oscillate above and below the static pressure. This phenomenon is referred to as a pressure swab when the pressure goes below static pressure, and a pressure surge when it goes above the static pressure. These are analogous to the well known water-hammer phenomenon, where the sudden closing or opening of a valve causes transient fluctuations of pressure within the pipe.

Transients occur both while pulling out of the hole and tripping in the hole. It is natural to associate a pressure surge with tripping in, and a pressure swab with pulling out. However, due to the nature of transient pressure, both surges and swabs could occur in either case. For instance, while pulling out, the string is first accelerated to the peak trip speed, held at that speed for some time, and finally decelerated to rest for each stand pulled. Although the pipe has been brought to rest at the surface, the elasticity of the string and the inertia of the fluid cause the pressure downhole to fluctuate between values below and above the static pressure, until these transients are damped out by fluid friction. Similarly, while tripping in, the pressures will fluctuate both above and below the static pressure due to the fluid transients in the hole.

Temperature Effects on Fluid Density

The density of drilling fluids is affected by the temperature and pressure at downhole conditions. The effective density at downhole conditions can be either above or below surface measured density depending on many parameters like oil or water based fluids, solids percent, temperature, downhole pressure, etc. Expansion of the fluid caused by increased temperature, results in a reduction in effective density. Compressibility of the fluid caused by increased hydrostatic head, results in an increase in effective density. While these two effects act in opposite directions to offset each other, one or the other can dominate, depending on well conditions. It is important to understand these effects when planning and drilling deep, high temperature wells.

Pressure Transients Due to Tripping In

The transient pressure effects while tripping in are best understood by following the events that occur during the trip-in of a stand of drillpipe, shown schematically in Figures 1-A - 1-F.

Initially, the pipe is at rest, the fluid is static, and hydrostatic pressures prevail in the fluid as shown in Figure 1-A. While tripping in, the pipe is accelerated to and held at a constant peak trip speed. During acceleration and tripping in, displaced fluid is accelerated, and it flows up the annulus and out at the surface.

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