Fracturing Aspects of Horizontal Wells

Abstract

Summary. This paper discusses the main reservoir engineering and fracture mechanics aspects of fracturing horizontal wells. Specifically, the paper discusses fracture orientation with respect to a horizontal wellbore, locating a horizontal well to optimize fracture height, determining the optimum number of fractures intercepting a horizontal well, and the mechanism of fluid flow into a fractured horizontal well. Introduction Interest in horizontal well drilling and completions has increased during the last few years. The significant advances in drilling and monitoring technology have made it possible to drill, guide, and monitor horizontal holes, making horizontal drilling not only possible but also consistently successful. Most wells have been completed as drainholes. These drainholes have been used in primary production and in EOR. Papers on drilling, completion, well testing, and increased production of horizontal vs. vertical wells have been presented in the petroleum literature. Many papers have dealt with steady-state production increase of horizontal wells over vertical wells Graphs and equations have been presented for calculating steady-state ratios for both fractured and unfractured wells. Ref. 2 provides a recent review of this technology. Other authors have studied the transient behavior of pressure response during a drawdown or a buildup of a drainhole. The literature lacks comprehensive studies on fracturing horizontal wells, and none of the studies cited above discussed this subject. Only Karcher et al. studied production increase caused by multiple fractures intercepting a horizontal hole. Using a numerical simulator, Karcher studied steady-state behavior of infinite-conductivity fractures. Stability of horizontal holes during drilling is another important aspect of horizontal well technology. It has been found that the degree of stability of horizontal holes depends on the relative magnitude of the three principal stresses and the orientation of the wellbore with respect to the minimum horizontal stress. Although productivity of horizontal wells could be two to five times higher than productivity of vertical wells, fracturing a horizontal well may further enhance its productivity, especially when formation permeability is low. Presence of shale streaks or low vertical permeability that impedes fluid flow in the vertical direction could make fracturing a horizontal well a necessity. This paper discusses fracturing horizontal wells from both reservoir engineering and fracture mechanics points of view. Our goal is to shed some light on important aspects of fracturing horizontal wells. Stress Magnitude and Orientation The first parameter to be determined is the fracture orientation with respect to the wellbore. Because fractures are always perpendicular to the least principal stress, the questions actually concern wellbore- and stress-orientation measurements. In what direction will induced fractures occur? What is the anticipated fracture geometry? What is the optimum length of the perforation interval? What is the optimum treatment size? What are the expected fracturing pressures? Data necessary for planning a fracturing treatment are the mechanical properties of the formation, the orientation and magnitude of the least principal stress, the variation in stresses above and below the target formaation, and the leakoff characteristics of the formation. It is commonly accepted that, at depths usually encountered in the oil field, the least principal stress is a horizontal stress. It also can be shown that the induced fracture will be oriented perpendicular to the least principal stress. The result is that a fracture created by a treatment will be in a vertical plane. If the horizontal segment is drilled in the direction of the least stress, several vertical fractures may be spaced along its axis wherever perforations are present. This spacing is one of the design parameters to be selected. Usually, this is investigated with numerical simulators. If the horizontal segment is drilled perpendicular to the least stress, one vertical fracture will be created parallel to the well. Figs. 1 and 2 show fracture direction vs. well direction. When the wellbore is not in one of these two major directions, several scenarios may occur, depending on the angle between the wellbore and the stress direction and on the perforation distribution and density. JPT P. 966⁁

A Numerical Study of the Effects of Packer-Induced Stresses and Stress Shadowing on Fracture Initiation and Stimulation of Horizontal Wells

“…These observations are detailed in Figures 2-1 and 2-2. The stability of horizontal wells depends on the relative magnitudes of the three principal stresses and the orientation of the wellbore with respect to the minimum horizontal stress (Soliman et al, 1990). In order to determine the magnitude and direction of the least principal stress (σ min ) the methods usually employed are the use of microfractures, long spaced sonic logging and strain relaxation (Daneshy, 1986).…”

“…In order to determine the magnitude and direction of the least principal stress (σ min ) the methods usually employed are the use of microfractures, long spaced sonic logging and strain relaxation (Daneshy, 1986). Inside the created fractures surrounding the horizontal wellbore, there would be a higher pressure drop than would be observed in a vertical well intersecting a vertical fracture with the same conductivity (Soliman et al, 1990). This is due to the increased pressure drop required to produce a well if the wellbore radius increases and infinite conductivity may exist inside a fracture for a predefined dimensionless wellbore radius.…”

Productivity Prediction of Fracturing Horizontal Gas Well Considering Fluid-Solid Coupling Effect

“…There are many prediction models for productivity of fracturing horizontal well: Ahmadreza [1] studied the numerical simulation model for productivity prediction of thin-interbed horizontal well to optimize hydraulic fracture interval; Weiyang X [2] established the productivity prediction model of shale gas fracturing horizontal well with multiple hydraulic fractures with consideration of the adsorption and desorption features of shale; Yang F [3] built the productivity prediction model of multi-section fracturing horizontal well with consideration of the impact from Non-Darcy's Flow; Soliman [4] did research on the early-stage productivity model of fractures with limited flow conductivity in a horizontal well of oil deposit with infinite thickness; M.J. Economides [5] proposed the equation to predict the pressure drop surround horizontal well bore based on steady-state liquid flow equation; Norris [6] showed typical production curve of horizontal well with multiple vertical fractures containing finite flow conductivity; Hegre [7] proposed the simple relations between effective well bore radius and fracture flow conductivity, fracture dimension, well bore radius, fracture quantity and fracture interval; Roberts [8] applied Non-Darcy's Flow model to conduct simulation evaluation on the productivity of multi-fracture horizontal well in tight gas reservoir; Soliman [9] proposed the productivity computation model of fracturing horizontal well under constant pressure in transverse fracture or longitudinal fracture. However, the above models mainly take the influence of hydraulic fracture into consideration and all assume there's no variation in physical property parameters or flow conductivity of fracture during the whole production process.…”

Modifying subterranean fluid flow patterns using barriers to improve pre-drainage in coal mining

“…A short hydraulic fracturing propagating longitudinally along the azimuth of the near wellbore is virtually assured as a result of the naturally fractured environment and initiation mechanics imposed in the near wellbore region by the introduction of a horizontal wellbore (Deeg, 1998;Soliman et al, 1990;Weijers et al, 1994). In the case of coal, a propagating hydraulic fracture dilates and extends pre-existing natural fractures and cleats whether the fracture is critically stressed relative to the maximum horizontal stress (Jeffrey et al, 1992;Jeffrey et al, 1995; and in one documented case the most unlikely angle to dilate, an intermediate direction of nearly 45-50° relative to the maximum horizontal stress (Johnson et al, 2010b).…”

“…The creation of singular and less complex fractures limits the near wellbore exposure of natural fractures to the FMM; not a desirable condition for a barrier placement. Cased hole, hydraulically perforated or slotted, shaped-charge jet perforated, or cemented frac sleeved completions are regularly used in stimulating horizontal production wells; these types of completions focus the frac fluid at selective locations to create longer, transverse fractures at regular spaced intervals and provide more efficient fracture length and proppant placement (Deeg, 1998;Soliman et al, 2008;Soliman et al, 1990). Maximising hydraulic facture length is completely contrary to the process of a barrier placement, as the goal is to disperse the fracturing energy into complexity along the entirety of the wellbore and dilate, treat and block off as many natural fractures and cleats as possible.…”