Tailored Pulse Loading Research Articles

The transient intrusion effect of high energy fluid on the rock failure around the wellbore during the dynamic stimulation process

Dynamic wellbore stimulation technologies, such as high-energy gas fracturing, electrical pulse fracturing, hydraulic pulsation fracturing, are generally designed to multi-fracture the near-wellbore region through generating high energy pulse with intermediate or high loading rate. During the stimulating process, the dynamic strength and failure mode of saturated rocks around the wellbore can be largely affected by the transient intrusion of high energy fluid. Accordingly, an experimental method was developed to provide a method to apply the tailored pulse loading in the central borehole of the cylindrical rock samples. Three groups (Saturated + Center hole exposed (S-CHE), Saturated + Center hole wall isolated (S-CHI), Dry + Center hole wall isolated (D-CHI)) tests were carried out. The borehole fluid pressure-time curves and the failure modes of rock samples were tracked, analyzed and compared for all of the experiments. Experimental results demonstrate that: The relationship between rock dynamic strength and loading rate is monotonic direct proportionality. The dynamic strength of saturated rock shows a much stronger strain rate dependence than that of the dry rock. At the same loading rate, fluid penetration can largely weaken the dynamic strength of rock samples. The stress pulse generates a complex crushed zone around the wellbore, while the fluid penetration tends to produce radial multi-fractures. These phenomenons were explained using the micromechanics-based macroscopic damage theory, which is capable of obtaining the relationship between the fluid intrusion and the fracture deformation in the rock dynamic failure process.

Effect of Multiple Hydraulic Fractures on Gas-Well Performance

Summary This paper describes the use of a single-phase 2D numerical model to study the performance of a well intersected by two perpendicular vertical fractures assumed to have either infinite or finite conductivities. Analysis of the simulated drawdown tests at a constant flow rate showed that the transient flow behavior of a well intersected by finite-conductivity (CfD<500) perpendicular fractures does not exhibit the bilinear and formation linear flow periods. However, when fracture conductivities were infinite (CfD =500), the formation linear flow period was observed. This period was used to determine the fracture half length and gave a fracture half-length equal to the sum of the two fracture half-lengths (xf + yf). The beginning of the pseudoradial flow period for any conductivity was found to decrease as yf/xf increased and to increase as the fracture conductivities increased for a given yf/xf. A single hydraulic fracture gave higher productivity than two fractures of the same total length when fracture conductivity was infinite. However, when fracture conductivity was low, the two fractures gave higher productivity than the single fracture. Introduction Hydraulic fracturing is an effective way to increase the productivity of wells producing from low-permeability formations. Considerable research has been conducted to determine the effect of hydraulic fractures on well performance and pressure-transient behavior. The results have been used to improve the design of hydraulic fractures. The increasing use of hydraulic fracturing to improve the productivity of oil and gas wells in low-permeability reservoirs has resulted in many research efforts aimed at increasing fracturing capabilities and evaluating fracture characteristics in the postfracturing period. The massive hydraulic fracturing (MHF) treatment is now a proven technique for developing commercial wells in low-permeability or tight gas formations. The purpose of MHF is to expose a large surface area of the low-permeability formation (in-situ permeability =0.1 md) to flow into the wellbore. Over the past few decades, hydraulic fracturing and explosive shooting of wells have been used to improve the productivity of tight reservoirs. An alternative to these techniques, one that uses certain features of both but appears better for creating multiple fractures, is tailored-pulse loading. Warpinski et al. in 1979 and Schmidt et al. in 1980 demonstrated that propellants can create multiple fractures in the field without damaging the wellbore region. Their results indicated that the long loading time of the pulse allows greater fracture extension and thus gives more opportunity for connecting a wellbore to the reservoir's natural fracture system.

Propagation of fluid-driven fractures in jointed rock. Part 1—development and validation of methods of analysis

We have developed a new method of analysis to describe the propagation of induced fluid-driven fractures in rock masses which contain pre-existing discontinuities such as joints, bed interfaces, lens boundaries, etc. The analysis is based on a 2-D model, with coupled solid mechanics, fracture mechanics and fluid mechanics. The solid and fracture mechanics are solved by an implicit finite-element approach which provides for mixed-mode (I and II) fracture propagation in arbitrary stress fields. The fluid mechanics capability first was established for steady-state conditions, using a finite-element formulation for flow between parallel surfaces. This initial coupling resulted in the version 1.0 of the FEFFLAP model (Finite Element Fracture and Flow Analysis Program). This version was verified against analytical solutions, and tested for validation against results of hydrofracturing in blocks of rock simulants containing an interface, under biaxial loading, as described in Part 2, the companion paper to this publication. The developments were then extended to the time-dependent domain by replacing the original fluid flow model with a model based on the FAST fluid dynamics module. The pressure profile inside the crack and the crack velocity are provided by a 1—D analytical formulation, corresponding to a constant-height fracture. Both arbitrary flow rate and borehole pressure conditions can be simulated; these may correspond, respectively, to conventional two-wing hydrofracturing and gas-driven tailored-pulse loading for multiple fractures. The model has been verified against analytical time-dependent solutions for fracturing in permeable and impermeable media. The new coupled model also has been validated against controlled physical tests involving hydrofracturing of blocks containing sandstone lenses, with the blocks loaded independently in three orthogonal directions. These tests are also described in Part 2 (Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 27, 255–268, 1990). These new analysis tools can be used for a wide variety of applications such as obtaining a better understanding of the stimulation of unconventional gas reservoirs, i.e. lenticular sandstones, coal beds and Devonian shales, or making improvements in the design of underground waste disposal by hydrofracturing or enhancing the fracturing og geothermal reservoirs.

Multiple fracturing of boreholes by using tailored-pulse loading : Swift, R P; Kusubov, A S Soc Pet Engr J, V22, N6, Dec 1982, P923–932

Multiple fracturing of boreholes by using tailored-pulse loading : Swift, R P; Kusubov, A S Soc Pet Engr J, V22, N6, Dec 1982, P923–932

Multiple Fracturing of Boreholes By Using Tailored-Pulse Loading

Abstract A laboratory experiment was developed to study the effects of pressurizing boreholes of rock samples with tailored pulses at intermediate loading rates. The experiment focused on ascertaining the loading conditions needed to initiate multiple-fracture growth from a borehole. Experiments were performed under controlled loading conditions with borehole pressures ranging from 10 to 100 MPa, loading rates from 1 to 100 MPa/ms, and pulse durations on the order of from 1 to 50 ms. The initial conditions of samples were varied to include dry, quasidry, and fully saturated states along with pressure confinement of the samples up to 50 MPa. Observations on samples of a medium-porosity sandstone showed that the degree of multiple fracturing is quite sensitive to the condition of the borehole as well as to the loading rate and appears strongly influenced by transient pore-pressure effects. The loading rate for the onset of multiple fracturing decreased with an increase in sample water content. The influence of water was much more marked at loading rates below 30 MPa/ms than at higher rates. For loading rates above the onset, the number of fractures initiated increased with an increase in loading rate, with about 10 to 12 at a loading rate of 100 MPa/ms. The fracture threshold-breakdown pressure increased proportionately with loading rate, a dependence that is attributed to transient water-intrusion effects around the borehole. The effect of sample confinement caused the threshold-breakdown pressure to increase and noticeably lowered the loading rate for multiple-fracture onset. Only two fractures (e.g., a typical hydrofracture) occurred when water was prevented from invading the borehole region. The tailored-pulse loading tests demonstrated their viability to determine the susceptibility of various rock types to multiple fracturing at intermediate loading rates.

Tailored-pulse loading conditions for multiple fracturing of boreholes : Swift, R P; Kusubov, A S Proc 22nd US Symposium on Rock Mechanics, Cambridge, Mass, 29 June-2 July 1981, P231–236. Publ Cambridge, Mass: MIT, 1981

Tailored-pulse loading conditions to produce multiple fracturing of boreholes are examined for sandstone samples having a dry density of 2.42 Mg/m/sup 3/ and 8.9% porosity. Experiments are performed with borehole pressures ranging from 10 to 100 MPa, loading rates from 10/sup -4/ to 10/sup 2/ MPa/ms, and pulse durations from 1 to 50 ms. Samples are tested for initial states of dry, quasi-dry, and total saturation, with confinement pressure varying from 0.1 to 50 MPa. Data show sensitivity of multiple fracture initiation on loading rate, water content, and confining pressure. The threshold-breakdown pressure increases proportionately with the loading rate. For quasi-dry and wet conditions, the loading rate for the onset of multiple fracturing decreases with increasing water content and with increasing confining pressure. When fluid intrusion around the borehole is prevented, only two fractures occur in dry samples, regardless of the loading rate or confinement conditions. These effects indicate that pore-fluid mechanisms, which result in a transient increase in pore pressure in the vicinity of the borehole, are responsible for multiple fracture initiation at intermediate loading rates. This influence of pore fluid suggests that higher permeable source rock may be more susceptible to multiple fracturing than low permeable rock. 6more » figures.« less