Model Investigation of Wellbore Pressure Distribution in Stem-induced Fracturing

Abstract

Model Investigation of Wellbore Pressure Distribution in Pressure Distribution in Stem-induced Fracturing Summary. This paper describes model testing conducted to investigate pressure variations along the length of a wellbore between a decoupled pressure variations along the length of a wellbore between a decoupled bottom charge and the stem. Tests were conducted in three-dimensional models made from thick plexigiass sheets that contained small decoupled explosive charges in the bottom of the borehole. Charge sizes varied from 75 to 200 mg, and the open borehole length between the top of the charge and the stem ranged from three to seven times the charge length. Both top and bottom charge initiations were investigated. Multiple pressure transducers were used along the borehore, and pressure/time distributions were determined at several locations. An additional series of tests is described in which the borehole geometry was altered by varying the diameter of an orifice plate placed in the wellbore. The use of this orifice plate was demonstrated to be effective in controlling the increase rate of the pressure pulse at the stem location. Introduction Tailored-pulse loading (TPL) is a new fracturing technique for oil and gas well stimulation. This technique was discovered under finding by the DOE Eastern Gas Shale Program (EGSP) and has been shown to be successful in the recovery of gas from Devonion shale reservoirs. With ordinary explosives it was postulated that pressure rise rates within the borehole were too rapid and had the net pressure rise rates within the borehole were too rapid and had the net effect of creating a stress cage. This stress cage was formed as a result of plastic deformation of the rock in the vicinity of the bore-hole. After passage of the outgoing, stress waves. the explosive loading, resulted in a compressive residual stress. This residual stress prevented the high-pressure gases created by the detonation of the explosive from entering the newly formed fractures and resulted in fracture lengths much shorter than desired. The stress cage also prevented the free flow of gas or oil back into the wellbore. prevented the free flow of gas or oil back into the wellbore. Hydraulic fracturing with very low pressure increase rates on the other hand, was found to be effective in creating only take long fractures, and these were always oriented in a direction perpendicular to the feast principal stress. TPL was seen to be a way of perpendicular to the feast principal stress. TPL was seen to be a way of creating multiple fractures of maximum length. In TPL. the increase rate is controlled to prevent a pressure stress cage from being formed but is high enough to ensure that multiple fractures are created. Schmidt et al. used TPL created by the use of propellants to demonstrate that the concept was feasible. These tests were conducted at the Nevada Test Site. More recently, propellants have been proven useful in increasing permeability and production in Meigs County under a program funded by the Gas Research Inst. However, propellants cost from 10 to 20 times as much per pound as propellants cost from 10 to 20 times as much per pound as conventional explosives, and the rate of deflagration depends strongly on confinement and therefore on the pressure. A second means of producing TPL was also identified under the EGSP. This technique uses ordinary explosives and is called stem-induced fracturing. Stem-induced fracturing uses conventional explosives and has been demonstrated to produce good wellbore fracturing and to increase oil production. In stem-induced fracturing, the explosive charge is located at the bottom of a borehole that has been drilled to a depth beyond the pay zone. Nothing is placed in the pay zone, and the hole is closed off with a packer or pedal basket located at the top of the pay zone. Several meters of crushed gravel are then used to complete the stemming to ensure that the stemming column will bridge in the hole when the explosive detonates. Upon detonation, the air shock and detonation products propagate up the borehole, and when they reach the stemming, there is a shock reflection that increases the pressure several-fold. The magnitude of this pressure and the rate of pressure increase have been found to be very pressure and the rate of pressure increase have been found to be very advantageous for well stimulation. The use of conventional explosives and stem-induced fracturing represents a considerable saving over propellants in TPL in terms of both material costs and implementation costs. Stem-induced Fracturing Stem-induced fracturing was identified in testing of models made of transparent plexiglass. These models were viewed with a multiple-spark-gap camera capable of framing rates of nearly 1 million frames/sec to investigate how fractures were formed. Fig 1 shows a typical model used in the testing. The 102-mm-thick models were placed in front of the camera so that the fractures initiated at the tips of the notches along the borehole length would be seen as a plane. The stem-induced fracturing uses a highly decoupled charge placed in the bottom of the borehole. The arrangement of the charge within the borehole is shown in Fig. 2. As Figs. 1 and 2 show, small piezoelectric transducers were used to monitor the pressures within the borehole and within the propagating fractures pressures within the borehole and within the propagating fractures (such as at Locations A and B). Four frames from a typical test are presented in Fig. 3. Plexiglass is a birefringent material, and the camera was fitted with Plexiglass is a birefringent material, and the camera was fitted with polarizing elements to permit high-stress areas to be identified during polarizing elements to permit high-stress areas to be identified during the testing. The black fringes in Fig. 3 represent lines of constant principal stress difference. A 250-mg charge was used for the test principal stress difference. A 250-mg charge was used for the test shown. Frame 5 (Fig. 3a) shows the location at 90 used of the fracture (the fracture is the dark bladder-shaped area near the bottom of the model) that initiated because of the detonation of the charge. Notice the two long, straight fringes located in the upper third of the model. These are caused by a shock wave that has traveled up the borehole as a result of detonation. The fringe makes an angle of 0.7 rad [40 deg the borehole wall, indicating that the ratio of shock wave speed to P-wave speed in the plexiglass is 1. 19. Note that there are two bright areas in the borehole: one near the bottom, where the charge was detonated. and one at the stem. The bright area near the stem is the result of an increase in pressure and temperature, which is caused by the shock wave reflecting from the stem. The light emitted is caused by the ionization of the air at that location. SPEPE P. 343