Experimental and Simulation Analysis of Jet-Perforated Rock Damage
- Asadi Mahmoud (Stim-Lab. Inc.) | Mogdeh K. Shirazi (Stim-Lab. Inc.) | Ali Ghalambor (U. of Louisiana at Lafayette) | Dan W. Pratt (Owen Oil Tools)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2001
- Document Type
- Journal Paper
- 176 - 181
- 2001. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 2.2.2 Perforating, 4.1.2 Separation and Treating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.3 Rock properties, 4.1.5 Processing Equipment, 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 5.3.4 Integration of geomechanics in models, 1.8 Formation Damage
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The finite-element method (FEM) is employed to evaluate stress distribution around a perforation tunnel with a sandstone formation. Experimental data are used to confirm the results of theoretical study. In the model, the effect of jet pressure, jet temperature, and the combination of both on formation are studied separately. The results indicated that a 3.5-g RDX charge is capable of disturbing rock formation up to 5 in. longitudinally and 2 in. radially. The extent of this damage zone for a 10.5-g RDX charge is 8 in. longitudinally and 4 in. radially. The linear effect is within the first few inches of the perforation tunnel entrance. In this region, rock grains are under an intensive sudden shock-pressure wave of 2 to 4 million psi for a fraction of a microsecond. As a result, the rock's physical structure will change to metamorphous with a lower bulk density than that of unshocked rock, causing a sudden decrease in formation porosity and, hence, formation permeability.
Formation damage caused by a perforation job has long been known, studied, and documented.1 Nevertheless, the severity of this damage, where rock properties alter, is an ongoing study in the form of experimental/theoretical investigations.
A shaped charge is a mechanically simple device with a complex operating procedure; the charge consists of a conical metallic liner, booster and main-load explosives, and a container or shell. Detonation of an explosive shaped charge creates an intense pressure pulse with an initial pressure and velocity of up to 5 million psi and 30,000 ft/sec, respectively. This highly focused jet-and-pressure wave strikes the rock formation and pushes aside rocks and pore fluids in its way to create a tunnel. As the shaped-charge jet penetrates through the rocks, it propagates both as longitudinal and transverse waves. Longitudinal waves can be compressive or tensile in stress and can penetrate both in solid rocks or formation liquids. Liquids, however, do not have shear strength and therefore do not transmit transverse waves. Shock-wave reflection caused by a shaped charge could, therefore, have a positive or negative influence on perforation penetration. Reflected tensile waves tend to help penetration, while reflected compressive waves can either increase or decrease it.
As the jet proceeds and the tunnel is formed, rock grains around the tunnel become compacted, altering the formation properties (i.e., porosity and permeability). This altered zone, also known as the crushed zone, is believed to have a porosity of up to 70% less than that of an unperforated formation.2 As the shaped-charge jet moves deeper into the formation, its energy decreases and has less effect on the formation properties. The effect of this alteration is shown in some cases to lead to an 80% reduction in well productivity.3-5 The extent of this damage, however, can be quantitatively evaluated with both the finite-element analysis and the principle of quartz deformation known as shock metamorphism.
Phenomenon of Shock Metamorphism.
A brief introduction to quartz crystal is given to better understand its behavior when subjected to high pressure/high temperature.Quartz Crystal.
Quartz is a crystalline form of silicon dioxide with a chemical formula of SiO2. Quartz is hard, brittle, and transparent, with a density of 2.649 g/cm3 and a melting point of 1750°C (3,182°F). Quartz undergoes different phase changes if subjected to high pressure/high temperature; it is categorized based on its behavior under temperature. A high quartz (known as beta quartz) is one that remains stable even above a temperature of 573°C (1,063°F). Crystalline of a low quartz (known as alpha quartz), however, when heated to a temperature above 573°C, changes to that of beta quartz with a hexagonal configuration rather than a trigonal one. In addition to crystal lattice change, the quartz crystal structure will also change when subjected to a high pressure of 2.5×106 psi or more. The tetrahedron arrangement of quartz around each atom of silica will therefore change to an octahedral form, called metamorphous.6Metamorphous.
When quartz crystalline changes its chemical structure because of high pressure, a new matter known as metamorphous will form. The critical shock-wave pressure at which quartz yields under a triaxial strain of a plane shock wave is called the dynamic elastic limit (DEL). If the imposed pressure exceeds DEL, the quartz crystal will undergo a series of deformations.7 Fig. 1 shows a plot of quartz deformation vs. imposed pressure. Because shaped-charge jet-perforation pressure does not exceed 5 million psi, one can conclude that the fracture element, shock lamellae, and stishovite, which are the first three phases of quartz deformation, will be the most probable outcome of a shock wave on sandstone quartz around a perforation tunnel. Recent studies have shown the existence of planar fractures around the perforation tunnel,8 as well as shock lamellae for a 10.5-g medium RDX shaped charge.9
According to Fig. 1, planar fractures represent the first quartz deformation phase when imposed pressure is higher than the DEL. This type of deformation is simply microfractures created because of a sudden low-to-medium pressure (i.e., pressure less than 2 million psi). At this stage, the chemical structure of planar fractures is still unchanged from that of unshocked quartz; therefore, its density remains the same.
As the imposed pressure is increased to 4.5 million psi, planar elements and stishovite may occur. Planar elements, also known as shock lamellae, form as a set of wavy parallel optical discontinuities on the surface of the crystal. Shock lamellae have specific, well-documented orientations occurring in multiple sets per quartz grain. Stishovite, on the other hand, forms when a phase transition caused by dynamic high compression occurs. Once this high-pressure pulse is released, the developed stishovite may revert to coesite, which is the next level of quartz deformation. Planar elements and stishovite are forms of metamorphous with different chemical structures and lower densities than those of unshocked quartz. At a greater pressure, however, quartz crystals are shown to form other types of metamorphous known as coesite and diaplectic glass (see Fig. 1). Once the imposed pressure approaches 8 million psi, quartz crystal melts.
The focus of this study is to use finite-element analysis to quantitatively map the jet-pressure profile around the perforation tunnel, both longitudinally (along the perforation tunnel) and radially (away from the shot center) as it moves into the formation and creates the tunnel. We will then compare it with the experimental results. The developed pressure-pulse contour map can then be related to the rock-grain deformation, using the phenomenon of shock metamorphism and, hence, rock-property alteration.
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