Development of formations with stress sensitivity is raising awareness that Geomechanics is a vital aspect of future management. Understanding geomechanical behavior is becoming more and more important for the petroleum industry. It has been reported by many authors (e.g. Herwanger and Koutsableloulis 2011) that, significant changes in pore pressure (ΔP) due to depletion or injection in weak formation might lead to increase in effective stress, compaction, reduction in porosity and permeability, casing deformation, failure and subsidence, challenges in fracturing the formation, closing and opening pre-existing fracture fault re-activation and bedding parallel slippage. The deformations affect the apparent time-shifts from seismic surveys of under- and overburden. The changes in stresses/strain affect the formation of interest as well as the overburden layers and directly affect all operations such as drilling, completion and production strategies because of permeability reduction. Stress affects nearly all petrophysical properties. Compaction, shear casing and well damage, cap-rock integrity, fault reactivation and sand production can occur during Formation depletion. A coupled Finite Element approach is taken for modeling Geomechanical effects induced by production/injection and the cycle dependence between pore fluid flow and def of the tight carbonate, which impact hydrocarbon production. Using Visage, the finite element analysis model for the geomechanical analysis and the fluid flow simulator Eclipse for ΔP determination, this work looks at ΔP – stress coupling, which has significant implications for porosity/permeability reduction. To address these issues, a conceptual two-way coupling model has been constructed using Finite Element method and Eclipse; the results show that, change in pressure has some implications on porosity and pereability. Both 3D and 4D Geomechanical models were developed that describe the state of stresses in the weak formation and overburden as well as changes in stress over time with either production or injection.
A clastic gas reservoir, have been characterized of being unconsolidated in nature, as well as its complexity and heterogeneity. Regarding to the ability to transmit fluids, this reservoir is predominantly tight with a wide variation in the permeability range, which confirms its heterogeneity. The depositional environment interpretation suggests the presence of channels running from West to East direction with kind of limited lateral extension. In terms of well completion strategy, most of the wells have been proppant fractured to produce at sustainable rates. Pressure transient analysis (PTA) has been applied extensively to better characterize and understand the reservoir behavior, well performance and to obtain significant properties of the reservoir. The challenges of this work consisted of performing a critical review of the existing PTA to identify some common behaviors or general trends in the pressure response. Considering the PTA have assessed the same reservoir and that several wells have been completed with similar completion techniques was expected finding similar pressure response behaviors. This paper presents different cases that have been analyzed through PTA, summarizing common trends, in terms of reservoir behavior and well response that have been observed during the study. The outputs of this study were used as inputs for the simulation reservoir models to narrow the uncertainty in some reservoir and wellbore properties and as inputs for the static model construction to better understand or confirm geological assumptions. The results of the PTA interpretation also helped to successfully drill newer wells away from any geological complexity and achieved the planned target of gas rates.
In acid stimulation treatments, acid will enter the most permeable or the least damaged zones. Most of the fluid will flow into the path of least resistance leaving large portions of the formation untreated. A critical factor to the success of an acid stimulation treatment is proper placement of acid so that all productive intervals are contacted by sufficient volumes of acid. The original stimulation fluid flow was altered to achieve uniform placement of treatment fluids during acid stimulation in candidate wells. Particle bridging technique utilizing self-degrading particulates of multiple grain sizes was utilized to achieve successful diversion and fluid placement across the entire interval of interest. Particle size distribution was calibrated for use with both near wellbore bridging across perforations and far field diversion inside wormholes and natural fractures. For self-degradable particulates to be successful as effective diverter, it should have accurate particle size distribution, therefore, a comprehensive well data analysis performed during design stage to recognize the opportunity for combination of far-field and near wellbore diversion systems for acid stimulation treatment. The combination of far-field and near wellbore self-degrading particulate diversion systems allowed the entire intervals to be treated evenly under matrix and fracture conditions, which is usually hard to achieve during acid stimulation treatments utilizing conventional chemical diversion systems, especially in cases where separate sets of perforations would need to be treated with a single stage. Evaluation of the diversion effectiveness was done by running temperature log immediately after the stimulation, which demonstrated satisfactory cool down effect across perforation intervals. This diversion technique was found to be more enhanced to effectively acid stimulate in high temperature carbonate reservoirs of Saudi Arabia. The utilization of self-degrading particulates of various grain sizes for far-field and near wellbore diversions during acid stimulation in high temperature carbonate reservoir was a unique approach and can be further optimized to resolve the challenges of multistage acid stimulation treatments.
Tight gas reservoirs and specifically low permeability gas-bearing carbonate formations require stimulation to produce commercial gas rates. Certain carbonate reservoirs in Saudi Arabia are highly heterogeneous and the degree of heterogeneity changes over the field and within well drainage areas. Horizontal multi-stage acid fracturing technology has become the preferred completion method in such reservoirs to stimulate these carbonate formations to maximize reservoir contact and enhance well productivity. Due to the degree of heterogeneity of these formations, a number of factors should be considered and a set of best practices should be implemented to overcome the reservoir challenges and achieve the expected results. Maximizing stimulated reservoir volume is one of the key aspects of the success of this technology. To maximize the stimulated reservoir volume, effective placement of the hydraulic fractures for every stage must be achieved, which is one of the most challenging factors. Effective placement of hydraulic fractures depends mainly on the following factors: placement of the horizontal wellbore along the minimum-stress direction and proper annular isolation to induce independent fractures across each stage. This goal becomes even more challenging to achieve with acid fracturing treatments. Failure to isolate the stages can lead to hydraulic communication between the fracture stages, allowing the pumped frac stage to propagate to the previously open frac stage, preventing the creation of separate fractures, affecting the well's productivity and ultimate recovery. A second factor that governs multi-stage completion effectiveness is the use of swellable packers to ensure proper isolation between the various stages, which was observed in the field. This paper describes the application of Multi-Stage Fracturing (MSF) completion technology. Field cases describing the deployment of the MSF completions and the obtained results after stimulation is presented, as well as the main conclusions and recommendations for successful implementation of this technology are highlighted.
Low permeability oil and gas-bearing carbonate formations are routinely completed with open hole horizontal laterals and multistage fracturing treatments to achieve maximum reservoir contact and enhance production of the formations. This completion technique has been successfully used in previously non-economical reservoirs — in North America and around the globe — to make them commercial. Open hole multistage fracturing technologies have been deployed in tight gas carbonate reservoirs to improve well productivity while using reservoir simulations to optimize the fracture design. When applying multi-stage fracturing one of the key aspects is to assure the isolation between the stages to induce independent fractures. This goal becomes even more challenging to achieve when the process is an acid fracturing treatment. Hydraulic communication between the fracture stages has been previously observed, which has prevented the creation of separate fractures transverse to the wellbore, thereby resulting in fewer and shorter fractures, mostly created along the wellbore plane. In many cases, new fractures could not even initiate and only matrix acid treatments could be conducted. To overcome the isolation challenge during an acid treatment, a multistage completion assembly based on sliding sleeves and swellable packers has been devised and implemented to conduct selective acid fracturing in low permeability carbonate reservoirs. This combined system has mechanically simplified the multistage fracturing job, reducing costs and complexity compared with traditional cement and perforating methods. The system provides excellent isolation between consecutive treatments by providing positive annular barriers. The technology ensures that the entire lateral is treated uniformly and according to the design, enhances proper fracture placement, increases reservoir contact area, and improves well productivity. This paper describes and addresses the successful deployment of this completion and fracturing technology. The success resulted from careful planning, fluid testing, and a comprehensive completion design that was fit-for-purpose to provide optimal stimulation treatment and productivity enhancement.
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