The problem of external pressurization and release is solved for a hollow sphere of incompressible material obeying Coulomb’s law of failure. The resulting relations between the applied pressure and the porosity of the sphere are used in formulating constitutive relations for the volumetric response of porous rocks. The theoretical predictions are compared with experimental data for a sandstone and a tuff.

With the current oilfield trend to pursue prolific oil and gas producers in the deepwater markets around the world; operators are drilling and completing larger boreholes to deliver and accelerate the production stream more effectively. Typically, in deepwater environments, the intervals to be perforated tend to be longer than on shelf-type completions. In addition, deepwater completions are more complicated with stacked pay zones that require additional completion steps and associated hardware. To execute these larger completions, operators normally opt to choose the largest perforating assembly that will yield the best charge performance (casing- entrance hole diameter and formation penetration) at in-situ conditions. These larger perforating systems with higher shot densities and total explosive load have the potential to create very large fluid-pressure waves and solid shock loads that can cause damage to the completion (unset packers, move bridge plugs, buckle tubing, split casing, etc.) under certain conditions. Because of the added expense of completing wells in the deepwater arena, problems during the perforation process can considerably increase completion expenditure. To help minimize potential risk during the completion program, it is critical to understand the dynamic pressure and shock events that occur during perforating. This paper will present a new numerical model that helps to ensure service quality by improving reliability on deepwater completions. The model has been validated with field measurements and employs high-speed recorders that were previously dedicated primarily to validating burn parameters for propellant-type equipment. Introduction The design of perforating assemblies up to this point in time has been based on simple rules of thumb such as: a 5 to 1 ratio of rathole volume to gun volume, standardized lengths of tubing (90 ft) between gun assembly and packer setting depth, or a minimum distance from the end of gun assembly to the well plug-back depth. Some have made arguments for stiff- versus-limber perforating assemblies and the implementation of shock absorption devices. The perforation process is normally a very safe step in the completion process when these simple rules of thumb are maintained. However, from time to time, an unexpected event such as bent tubing, collapsed spacer gun, failed packer/bridge plug, collapsed casing, etc. will occur during the perforation event even though the rules of thumb are followed. Standard root-cause analysis is to review the procedures that were executed and analyze the metallurgical properties of the failed components to ensure that no flaws due to corrosion or fatigue were present. Typically, the job parameters are compared against other jobs with similar job specifics to show how many similar jobs have been performed without incident. At the end of the post-job analysis process, if no specific cause is found, the standard response has been that this type of perforating job has been executed a given number of times with a given success rate as reported in a database. All equipment typically is observed to be in acceptable working condition (tensile, burst/collapse ratings, etc.), and the job is classified as a random event with an unknown root cause. The authors suspected that most of these supposedly random events actually were due to root causes that were beyond the capability of the technology available at the time to detect and analyze. In order to understand the responsible phenomena for these incidents more clearly, a research project was launched to quantify the dynamic perforating events with special high-speed downhole memory recorders that sample pressure, temperature and acceleration. The high-speed recorder measurements on actual field perforating jobs were then used to characterize and validate a physics-based numerical model. The concept of applying physics-based numerical models to explain dynamic behavior during perforating events has been applied previously1 and has been used specifically to characterize combined perforating/ propellant burn events.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPropellants, now sometimes combined with perforating guns, have the potential to clean up damaged perfs, penetrate nearwellbore damage, and create formation fracturing. Although some of these dynamic techniques have been available for years, the lack of a standard measurement and analysis method has restricted their widespread use. Now, improved ultra high-speed downhole memory data recorders and computer software fully incorporating well, perforation, and tool details address this issue. Hardware and software are described. Several demonstration cases show the use of this technology in the field for vertical and deviated cased and open hole wells. Dynamic pressure data are compared with calculations and the realism of the results is shown, as well as how to make procedural and design improvements. The combination of the new recorder and software for the design and evaluation of dynamic well treatments allows the optimization of job design, and the evaluation of post-job results to understand tool performance and improve future tools and applications.

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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