The low recovery and high oil volume remaining in shale oil reservoirs are a strong motivation to investigate the application of enhanced oil recovery methods in these reservoirs. This paper presents the potential of applying cyclic CO 2 injection to improve the recovery factors of shale oil reservoirs. Cyclic CO 2 injection could be an effective technique to improve the oil recovery of this type of reservoirs for several reasons. It is a single-well process; well-to-well connectivity is not required, the hydraulic and natural fractures provide a large contact area for the injected gas to penetrate and diffuse into the lowpermeability matrix; and the payback period of the cyclic CO 2 injection process is short compared with the other flooding process. Very limited numerical and laboratory studies are available to study the feasibility of CO 2 huff-n-puff for shale oil reservoirs. Latest numerical studies have revealed that CO 2 huff-n-puff technique could be an effective method to increase recovery factors of shale oil reservoirs. In order to support the numerical studies results, a laboratory study was conducted using shale cores from Mancos and Eagle Ford. The aim of this study is to evaluate the potential of cyclic CO 2 injection. Many design parameters such as soaking period, soaking pressure, and numbers of cycles were considered to evaluate the feasibility of cyclic CO 2 injection. The laboratory results indicate that cyclic CO 2 injection improved recovery of shale oil cores from 33% to 85% depending on the shale core type and the other operating parameters. These results have shown that cyclic CO 2 injection is a promising method to improve the recovery of shale oil reservoirs. Also this study aided to develop a better understanding of the performance of cyclic CO 2 in shale oil reservoirs.

Low oil recovery in shale oil reservoirs and vast shale reservoir volumes stimulate our efforts to investigate the application of enhanced oil recovery methods in shale oil reservoirs. A recent numerical study has indicated that cyclic gas injection could be an effective method to increase the oil recovery of shale oil reservoirs, and gas channeling can be mitigated. This paper presents our experimental verification and quantification of the potential to improve oil recovery by cyclic gas injection in shale oil reservoirs. Core plugs of Barnett, Marcos and Eagle Ford shales were used. The oil used was Mineral oil (Soltrol 130) and the gas used was Nitrogen. Unfractured cores were used in the experiments. The effects of cyclic time and injection pressure on oil recovery, among other parameters, were investigated. Our results also showed that cycle gas injection could increase the recovery from 10 to 50% depending on the injection pressure and shale core type. This study shows that one of the important mechanisms of cyclic gas injection is the pressure effect that causes a large pressure drawdown during the production phase. The cyclic gas injection provides an effective and practical method to improve oil recovery in shale reservoirs because the gas needed is available in liquid-rich shale plays.

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⁁

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In recent years, new fracturing designs and techniques have been developed to enhance production of trapped hydrocarbons. The new techniques focus on reducing stress contrast during fracture propagation while enhancing far field complexity and maximizing the stimulated reservoir volume. Zipper frac is one of these techniques, which involves simultaneous stimulation of two parallel horizontal wells from toe to heel. In this technique, created fractures in each cluster propagate toward each other so that the induced stresses near the tips force fracture propagation to a direction perpendicular to the main fracture. The effectiveness of zipper frac has been approved by the industry; however, the treatment's optimization is still under discussion. In this paper, we present a new design to optimize fracturing of two laterals from both rock mechanic and fluid production aspects. The new design is a modification to zipper frac, where fractures are initiated in a staggered pattern. The effect of well spacing on the changes in normal stress has been evaluated analytically to optimize the design. Results demonstrate that the modified zipper frac improves the performance of fracturing treatment when compared to the original zipper frac by means of increasing contact area and eventually enhancing fluid production.