Use of Pressure-While-Drilling Tools to Improve Formation Integrity Test Interpretation

Abstract

In deepwater drilling, a small margin usually exists between formation integrity and pore pressure. Accurate determination of the formation integrity at casing shoes is paramount to drilling an interval without losing returns. Non-aqueous fluid is often the fluid of choice for deepwater environments, and cold temperatures in the riser result in higher drilling fluid viscosity and gel strengths along with pressure and temperature effects on fluid density. These effects on fluid properties can induce inaccuracies in the interpretation of surface collected data when determining the fracture initiation pressure (FIP) during a formation integrity test (FIT). Downhole pressure technology can be used to more accurately determine fracture initiation during a FIT by measuring the true bottomhole pressure at the formation, thereby eliminating the effects of mud gel strength and varying mud weight in the mud column. The comprehensive pressure data, however, is stored in downhole memory and can only be accessed upon tool recovery or wireline data link. Neither of these options are economically attractive in the deepwater cost environment. However, limited downhole pressure data can be transmitted back to the surface once circulation is re-initiated. This paper presents a field useable method which makes use of limited real-time downhole pressure data to correct interpretation of surface collected data. This method has been compared to over 20 FIT results from actual downhole memory data and yields a close match. Introduction Drilling in deepwater has become commonplace in most oceans and seas of the world. One characteristic of deepwater drilling that has grown more acute with increasing water depths is the ever-diminishing margin between pore pressure and fracture gradient. This "window" between pore pressure and fracture gradient is often the principal well design factor and affects well cost and project economics. During deepwater drilling operations, the operating window is further reduced by the mud weight needed to maintain well control, circulating friction, and suspended cuttings. For most deepwater exploration wells and many deepwater development wells, a FIT is run at each casing shoe to verify the fracture gradient and available "window" or margin between the mud weight and the fracture gradient. A FIT is also known as a pressure integrity test (PIT) or as a leakoff test (LOT). The margin between mud weight and FIP is an important quantity to the drill team. While circulating the well, flow of drilling fluid up the annulus induces frictional pressure losses which act upon the hydrostatic mud column and hence the openhole interval. Since the cuttings are denser than the drilling fluid, the suspended cuttings also act to increase the effective density of the mud column. The combination of static mud weight, frictional pressure losses in the annulus, and drill cuttings being carried out of the well is known as equivalent circulating density (ECD). The margin between mud weight and FIP must be sufficient to accommodate these additional pressures / stresses or the formation will fracture and fluid losses will result. Loss of returns leads to increased fluid costs and non-productive time (NPT). An accurate FIP is required to mitigate these events and maintain well control. In addition to managing the ECD, the FIP is important for calculating kick tolerance and determining the ability to increase mud weight. In deepwater drilling, operating margins between mud weight and FIP are generally less than 1.0 lbm/gal (ppg). The advent of Pressure-While-Drilling (PWD) tools have allowed accurate determination of downhole pressure conditions and ECDs. Despite this advance in the measurement of downhole pressures, incidents of lost returns have occurred in deepwater operations that have defied simple explanation, i.e. losses have occurred at measured downhole pressures less than casing shoe FIPs. The suspected root cause of many of these incidents was the practice of comparing PWD ECD measurements to FIPs derived from surface data measurements. This paper describes a technique developed to improve the accuracy of FIP determination through use of PWD data combined with surface collected data. This allows reducing downhole circulating pressures by controlled drill rates and lower circulation rates yet maximizing the allowable rate of penetration (ROP) while avoiding lost returns.