Fracture Pressure-Slope Analysis for TSOs in High-Permeability Formations
- Jeffrey E. Smith (Chevron Oil Co.) | Bruce R. Meyer (Meyer & Associates Inc.) | R. Henry Jacot (Meyer & Associates Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 2002
- Document Type
- Journal Paper
- 110 - 121
- 2002. Society of Petroleum Engineers
- 3 Production and Well Operations, 2.4.6 Frac and Pack, 4.1.2 Separation and Treating, 5.5.8 History Matching, 2.2.2 Perforating, 4.3.4 Scale, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.5 Reservoir Simulation
- 0 in the last 30 days
- 436 since 2007
- Show more detail
- View rights & permissions
|SPE Member Price:||USD 5.00|
|SPE Non-Member Price:||USD 35.00|
The relative popularity and success of the frac-pack technique in hydraulic fracturing has resulted in some misconceptions regarding the objective, procedure, and pressure analysis after a screenout. This paper addresses frac-pack procedures and the pressure response after a tip screenout (TSO).
An analytical method has been developed for analyzing pressure- slope behavior after a TSO in high-permeability formations. These equations incorporate the first order parameters affecting the fracture pressure, rate of pressure change (derivative), and pressure- slope behaviors after a screenout.
The fundamental equations for pressure-slope analysis are similar to those originally developed by Nolte for pressure-decline analysis. The major difference is that after the fracture stops propagating (i.e., after a TSO), the injection rate is not zero. Consequently, if the injection rate is greater than the leakoff rate, the fracture volume and net pressure (constant compliance) must increase. If the injection rate falls below the leakoff rate, the fracture net pressure must decrease.
Although analytical equations will not replace 3D fracturing simulators normally used for design and real-time history matching, they do provide insight into the major parameters affecting pressure behavior after a TSO without running a numerical simulator. The analytical equations presented in this paper demonstrate why pressure slopes after a screenout are typically much greater than unity for low-efficiency fractures.
A generalized set of equations is presented for analyzing the pressure-slope behavior after a screenout. Numerous graphs are provided that illustrate the parametric effects of fracture efficiency, spurt loss, and fracture net pressure at the time of a screenout on the pressure, derivative, and slope behaviors after a TSO. Comparisons of the analytical pressure-slope equations with a 3D fracturing simulator are presented to show the analysis' application.
A new methodology of frac-pack post-analysis is presented using the pressure slope technique. This methodology uses the pressure slope during a screenout as a check on the minifracture and fracture efficiency. Two frac-pack cases with bottomhole data are analyzed with a 3D hydraulic fracturing simulator to illustrate the pressure-slope analysis for low efficiency fractures.
Godbey and Hodges1 recognized the importance of analyzing fracture- pressure data during a hydraulic fracturing treatment in 1957. However, it wasn't until 1979 that Nolte2 developed a classic method of pressure-decline analysis (fracture calibration or minifracture analysis) for estimating the closure pressure, efficiency, leakoff coefficient, and fracture geometry.
In 1981, Nolte and Smith3 presented a technique for interpreting fracturing treating pressures based on the "Mode" of the pressure slope. The unit slope, or Mode III, behavior in log-log space implied "that the incremental pressure change is proportional to the incremental injected-fluid volume." The interpretation is "that a unit slope implies that a significant flow restriction has formed in the fracture (e.g., proppant screenout)." Although Nolte and Smith pointed out that the net pressure increases at a rate proportional to the net injection rate, their analysis to find the screenout location was based on a fracture efficiency of unity.
Smith4 presented a paper in 1984 identifying the method of "a controlled screenout to achieve enough propped fracture width to ensure lasting fracture conductivity." The controlled terminology distinguishes itself as a designed screenout rather than one that occurs inadvertently.
In 1988, Nolte and Economides5 presented a chapter in which they identified that "a log-log slope approaching one indicates restricted fracture extension at the fracture's extremities and the requirement for a larger pad; whereas a slope greater than one indicates a restriction within the fracture. . . ." This statement, however, was based on the understanding that the fracture efficiency was near unity.
Nolte6 presented another paper in 1990 addressing fracture pressure-analysis deviations from idealized assumptions. This paper contains the first presentation of an idealized formulation of pressure/width relationships after a TSO in terms of a net pressure log-log slope by adding the volume of fluid injected after the screenout to his original analysis.
In 1996, Valkó7 presented a method for "determining the packing radius corresponding to a width inflation period" with the pressure derivative and instantaneous compliance factor for the radial fracture-geometry model. Their frac-pack example was for a fracture efficiency greater than 60%. The behavior of their bottomhole treating pressure was very atypical of the low-efficiency (?<5%) cases addressed in this paper. In general, the fracture efficiency must be low to achieve a propped pack.
TSO and frac-pack designs have gained increased popularity in the industry, especially in high-permeability formations where greater conductivities are needed to increase production rates or where formation damage is a problem.
The TSO technique is used deliberately to create a proppant screenout or bridging condition around the perimeter of the fracture to prevent further propagation and height growth. Continued pumping results in "ballooning," or increasing the fracture aperture with continued increasing fracture pressure. Typically, only the fracture perimeter is packed.
Frac packs differ from TSOs by packing the entire fracture with proppant from the tip to the wellbore, which greatly enhances the fracture conductivity. This technique is typically performed in high-permeability formations that require higher fracture conductivities (i.e., to obtain an FCD>2).
The relative popularity and success of the frac-pack technique in hydraulic fracturing has resulted in many misconceptions regarding the objectives, methodology, and procedures for these unconventional treatments. One of the misconceptions is that the pressure slope during a screenout is unity and that slopes greater than unity indicate a flow restriction in the fracture closer to the well (i.e., other than at the fracture tip). This paper clearly shows that the pressure slope is greater than unity for low-efficiency fractures during a TSO.
|File Size||862 KB||Number of Pages||12|