In-Situ Stress Measurements During Drilling

Abstract

Summary This paper describes the results of six microfracturing experiments in a gas well in south Texas. The experiments were conducted in open hole and during the drilling operations. Microfracturing consisted of pumping very small volumes of drilling mud (tens of gallons) at very low rates (3 to 30 gal/min [189 to 1892 X 10(-6) m3/s]). Three of these microfractures extended below the bottom of the open hole and were cored out. Created fracture orientation was obtained from the fractures observed in the oriented core. Several instantaneous shut-in pressures recorded in each zone showed variations of about 200 to 300 psi [1.4 to 2.1 MPa]. This magnitude change is attributable to heterogeneity of the rock. Measured values of instantaneous shut-in pressure (ISIP) did not show any trend with lithology (shale or sand-stone), mechanical properties, or tensile strength. properties, or tensile strength. Introduction The experimental work described in this paper answers two questions.In a given field, what is the orientation of induced hydraulic fractures?In a given field, is there enough stress contrast between adjacent zones for fracture containment? The method used is quite simple. Create a hydraulic fracture in the open hole while the well is being drilled. There is a good chance that the fracture will extend below the bottom of the hole. Core the bottom of the hole. Obtain fracture orientation by observing its direction in the oriented core. The advantages of this method of operation are that all the data are obtained very early in the life of the reservoir. Openhole fracturing eliminates the effects of casing and perforations on fracture orientation and pressure. Coring the fracture provides a very positive visual determination of fracture orientation. The petroleum literature contains many fine articles on the relationship between in-situ stresses and fracture height, the need for determining fracture orientation, and how such data can be used for optimizing reservoir performance. For the sake of brevity, this paper will not performance. For the sake of brevity, this paper will not include a literature survey or a discussion of why the experiments were conducted. The major emphasis will be on a more complete description of each operation and results. Veatch provides background information on hydraulic fracturing. Experimental Procedure All the experiments were conducted in a gas-producing well in south Texas. The well has two producing zones separated by shales. The experimental work consisted of creating microfractures in each producing zone and in the shale zones above, between, and below the zones. Oriented cores were also obtained from every fractured zone. These cores were tested for physical and mechanical properties as well as for determination of actual fracture properties as well as for determination of actual fracture orientation. The sequence of events for each microfracture was as follows.With the drillpipe out of the hole, the openhole packer was assembled. This assembly contained two Bourdon tube (BT) gauges that would record the fluid pressure in the drillpipe (tubing). The packer was attached to an anchor pipe. By changing the length of the anchor pipe, one can regulate how high above the hole bottom the packer will seat, thus regulating the height of the openhole section to be fractured.The packer was lowered into the hole with the drillpipe. Once on bottom, it was seated by application of a vertical force to the drillpipe.The surface lines between pumps and wellhead were pressurized to detect and repair any leaks. Next, the pressurized to detect and repair any leaks. Next, the drillpipe was pressurized to ensure proper seating of the openhole packer. A small pressure was also applied to the annulus to detect and repair any leaks.Microfracturing was done by pumping of drilling mud as the fracturing fluid. Specific details of microfracturing will be discussed later.After microfracturing, the packer was released and brought to the surface. BT gauges were recovered for later determination of actual bottomhole pressures (BHP's).An oriented core was cut from the bottom of the hole for physical and mechanical rock-property measurements, as well as for detection of fracture orientation. The following sections provide more detailed descriptions of some key operations. Wellbore Preparation. The hole was drilled and cased to a depth of 7,750 ft [2362 m] according to standard oilfield operations. P. 891