In situ mineback experiments were conducted at DOE's Nevada Test Site to evaluate the accuracy and reliability of in situ stress measurements made with small hydraulic fractures in cased and perforated holes. The major problem encountered is crushing, damage due to too large of a perforation charge. Too small of a charge can also reduce the accuracy of the measurement because of poor penetration. Phasing of the perforations did not appear to have a large effect. From these results an optimum perforation schedule is suggested. The pressure response during the stress test is found to provide important diagnostic information on the quality of the measurements. Examples from stress tests at 8000 ft depth in sandstones and shales are given for comparison
INTRODUCTION
The state of stress at depth in the earth has considerable significance for many oil and gas exploration, stimulation and production processes. For stimulation technology in particular, the in situ stress distribution is the principal factor that controls hydraulic fracture orientation and height and it may have a significant influence on long term production by its effect on proppant crushing, embedment, and reservoir properties. Knowledge of the in situ stress state should be a high priority requirement for optimum fracture design.
This study is mainly concerned with the determination of the vertical distribution of the minimum, principal, compressive, horizontal, in situ stress (hereafter referred to as the minimum in situ stress, smin) because of its overwhelming effect on hydraulic fracture height. Perkins and Kern1 were the first to explain how in situ stress differences between the pay zone and the bounding layers can restrict fracture growth. Simonson et al2 demonstrated how to calculate fracture height in a non-uniform stress field for an equilibrium crack and Cleary3,4 developed artificial spreading rate criteria for fracture height growth which could also be used in non-equilibrium conditions. These techniques can be very useful for fracture design procedures, but the vertical distribution of the minimum stress must first be known -- either by prediction or measurement.
The total in situ stress state at depth is very difficult to predict because of the many factors which may influence its magnitude and orientation. Some of these factors are depth, pore pressure, rock properties, tectonics, topography, temperature, fracturing and residual stresses. Separating the effects of these various parameters becomes extremely complex. Nevertheless, Hubbert and Willis 5 were able to demonstrate very simply that the horizontal stresses can range from about one third of the overburden stress to three times the overburden stress. Haimson 6 reviewed various measurements of in situ stresses; these measurements agreed with the range of values predicted by Hubbert and Willis and were usually found to be quite consistent over a large area. Haimson6 reviewed various measurements of in situ stresses; these measurements agreed with the range of values predicted by Hubbert and Willis and were usually found to be quite consistent over a large area.
In oil and gas applications, it is very difficult to perform stress measurements in wells which are dedicated to stimulation and production
