Accurate In-Situ Stress Measurements During Drilling Operations

Abstract

SPE Members Abstract This paper describes easily implemented field procedures for obtaining, in situ horizontal earth stresses during drilling operations. The procedures were developed to obtain stress data that are necessary for wellbore stability modeling and for predicting and avoiding fracturing lost returns. Conventional leak-off tests are performed routinely to test the integrity of casing cement jobs, not to accurately measure earth stresses. The most important consideration was to develop methods that were easy to implement and would minimize disturbance to drilling operations, thereby promoting acceptance among operations personnel. The procedures are called Extended Leak-off Tests (XLOT). Introduction Over the past several years, increasing effort has been directed toward developing methods to preserve wellbore stability during drilling, particularly in water sensitive, weak shales. Early research focussed on the chemical aspects, but recognition of the chemical and mechanical interactions that take place during drilling is increasing. In this framework, drilling fluid density is an important design element, and a knowledge of earth stresses and rock strength is required. Linear elastic equations for calculating stresses at wellbore walls are well known (Hubbert and Willis 1957, Bradley 1979), but were limited in their application by the absence of data on the in situ strength of shales. This began to change with the development of methods for determining downhole shale strength from cuttings analysis (Steiger and Leung 1990). The equations can now be applied quantitatively to design drilling fluid densities that stabilize inherently weak shales. This modeling approach requires cuttings analyses, pore pressure and stress data to produce a chart that prescribes stable wellbore mud weight vs. depth (Figure 1). Widespread application of this technology within Exxon prompted the development of improved field methods to obtain the stress data necessary for computer modeling of wellbore failure. Extensive research has been dedicated to horizontal stress measurement in wells (Haimson 1988). Methods include microfracturing (Fairhurst 1968), as well as more complicated multiple fracture generation (Cornet and Valette 1984) and holographic strain relaxation measurements (Smither, Schmitt and Ahrens 1988). However, the consensus is that low volume hydraulic fracturing (microfracturing) is still the most practical and reliable method for deeper wells. We sought to obtain higher quality data for stress analysis by modifying pressure integrity tests (PIT's) to incorporate current fracturing principles. Pressure integrity tests taken to leakoff at freshly cemented casing shoes are essentially microfracturing tests, although the data are often of insufficient quality to allow measurement of stresses. In order to promote acceptance of the modified procedures, a key concern was to minimize disturbance to drilling operations. Conventional Leakoff Tests P. 491^