Laboratory, Computer Modeling, and Field Studies of the Pulse Fracturing Process

Abstract

A combined testing and modeling program has been completed with the purpose, of providing systematic observations and models of the pulse fracturing process. Approximately eighty individual laboratory process. Approximately eighty individual laboratory tests have been successfully carried out in blocks of scaled rock simulant loaded to a true three-dimension stress state with miniature propellant/explosive charges in a wellbore as the fracturing energy source. Real-time borehole pulse pressure measurements were also obtained. Mathematical models developed include descriptions of borehole pressurization, flaw initiation, gas-driven fracture propagation, and fracture arrest. Measurable material and natural parameters such as moduli, gas permeability, and in parameters such as moduli, gas permeability, and in situ stress are used in the models. Models include true crack-crack interaction. One of the models is fully two-dimensional and includes fracture curvature in an anisotropic stress field. Results indicate that multiple fractures are created over a broader range of source, material, and natural conditions than had commonly been thought before, but that the conditions conducive to extremely long fracture extension, usually considered desirable, are somewhat difficult to establish and maintain. Several approaches to optimizing and evaluating the process in the field are suggested, including a possible approach to the generation of long fractures. A field test observation is described. Introduction Pulse fracturing, or multiple radial fracturing, has Pulse fracturing, or multiple radial fracturing, has long been considered as a possible method of well stimulation that might be useful in certain special cases, such as the stimulation of a fractured reservoir under conditions where a conventional hydraulic fracture is not economic or not likely to be oriented such as to intersect the natural fracture system. More recently, the method has also come into some use as a means of clean-up or skin reduction. In most commercial applications, some type of propellant charge is ignited at the desired depth in the wellbore to create a source of gas pressurization at a rate intermediate between the slow rate used in conventional hydraulic fracture and the extremely high rate generated by explosives. Although the concept has been shown to be viable, making quantitative interpretations of tests to date have been difficult. This suggests that the method is probably sensitive to several parameters of the rock and stimulation tool system that are difficult to quantitatively understand. The study described here (Schatz et al. 1987) is quite comprehensive and was intended to determine under what conditions pulse fracturing is a practical approach. To do this, the parameters that govern the occurrence of pulse fracture initiation and the maximum distance that fractures can be driven into the formation were determined. The study is broken down into experimental and theoretical parts, with some field observations. LABORATORY RESULTS Laboratory experiments have resulted in a successful method of scale-modeling the pulse fracturing process for use in comparisons with theoretical and field results. The experiments were conducted in a true polyaxial stress state on blocks of material that polyaxial stress state on blocks of material that measured approximately 12 × 12 × 15 inches with a central 3/8 inch diameter simulated wellbore, as shown in Figure 1. In the wellbore, a small propellant, explosive, or special combination charge was initiated. Dynamic wellbore pressure was measured, and post-shot fracture geometry was observed for comparison with theory. More than 80 successful experiments were run. P. 375

Numerical Modelling and Parametric Analysis for Designing Propellant Gas Fracturing

“…Numerically, fluid discharge is calculated by solving the differential equation group of Equation (19) and (20).…”

“…The flow may be a combination of laminar flow and turbulent flow. The total pressure drop is expressed as 19 :…”

“…In order to understand the mechanism of propellant gas fracturing, Sandia National Labs in USA conducted a series of lab and field experiments [14][15][16] in 1980's. In the same period, Nilson and Schaltz also published their results, respectively, on simulating propellant gas fracturing numerically [17][18][19] .…”

Experimental Study on Fracture Initiation by Pressure Pulses

“…It has been indicated that there exists a correlation between the critical pressurization rate and the tensile strength of rock. If the data of the test series #2 and #3 are plotted together, as in Figure 10, the two parameters obviously present a linear correlation, which can be described as the following equation: ( 3 ) in which, c dt dp       stands for the critical pressurization rate for the transition from the hydraulic-fracture pattern to the multiple-fracture pattern, and 0 T is the tensile strength.…”

“…The experimental studies normally employ small samples and only examine the post-shot fracture geometry instead of recording the whole process of fracture development. The problem of dynamic fracturing was previously studied by Schaltz et al 3 Their experiments were conducted in a true three-axial stress state. The samples were approximately 15 12 12 × × inches with a central simulated wellbore of 3/8 inch in diameter.…”