TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractBall sealer diversion has been proven to be both an effective and economic way to selectively stimulate low permeability oil and gas reservoirs in hydraulic fracturing and matrix acidizing treatments. However, the design and implementation of a successful ball sealer diversion treatment is still a challenge. Often the designer depends on experience, and lacks the knowledge of accurate ball transport and sealing behaviors. An integrated model for selecting operating factors such as fluid and ball properties, as well as predicting the ball sealers transport and hydraulic behavior prior to pumping is needed for optimizing the stimulation process.In this paper, an integrated transport model is presented to describe the relationships among the ball sealer transport sealing behavior, wellbore deviation, wall effect, perforation density and size, fluid properties, pumping rate and ball properties. In addition, the smoothness of ball, perforation phasing, and velocity profile inside the wellbore during ball seating are also taken into consideration. Recommendations are provided for determining the number of ball sealers per job for either single or multiple stage treatment, the designed pumping rate, and the physical properties of the fluid and ball sealer. A hydraulic analysis model is presented for the overall fluid dynamics starting from surface, through wellbore, to reservoir. This analysis describes the effects of reservoir condition, pressure drop on perforations, and actual sealing efficiency on the surface treatment pressure profile. This paper will investigate the effects of the diversion factors on the ball transport behaviors such as transport time, ball sealer efficiency and surface pressure.
Mud removal and cement placement during a cementing operation are key factors to ensuring zonal isolation. Actual well testing results show that majority of wells have zonal communication during the life of production. The communication between water and oil zones may significantly affect oil production and require expensive remedial squeeze treatments. Fully understanding the flow characteristics and interactive behaviors in mud, spacer, and cement is an important step to ensure critical zonal isolation.A newly developed computational fluid dynamics model helps end users better understand the transport phenomena of intermixing multiple fluids. Fluid decay resulting from the intermixing involving the mud, spacer, and cement systems is quantified for given downhole conditions of wellbore geometry, fluid properties, pump rates and casing centralization. The robust method allows the analysis of potential hydrocarbon production zonal isolation success and optimization of cement placement. This advanced fluid displacement simulator has been field verified with impressive results for a wide range of annuli. A recently developed pseudo 3-D visualization module aids in understanding the complex phenomena as well.Some field cases used for verification are included. The detailed job analysis demonstrates the methodology used to study the effects of fluid systems, pump rates, and centralization configurations and provides application engineers the opportunity to understand different scenarios while optimizing key parameters to achieve top tier results.
The quality of zonal isolation and well integrity are two main objectives for a successful cementing job. These objectives require proper placement of cement in the annular depth interval of interest. Cement placement, in turn, is dependent on effective drilling mud removal. A spacer fluid is designed to aid displacement of the drilling fluid and to minimize cement contamination. It takes into consideration not only wettability tests and compatibility results between spacer/mud and spacer/cement, but also rheological properties to improve friction pressure hierarchy. The rheological properties vary depending on flow rate, polymer concentration, spacer density and temperature. Trial-and-error lab tests and re-designs are needed to establish a spacer to meet those requirements. To optimize the costly process, and to expedite the job design, it is necessary to propose a smart rheological hierarchy optimization methodology that can predict the spacer rheologies and polymer concentrations by leveraging information from known data. In this paper, a novel rheological hierarchy optimization methodology with artificial intelligence and inverse technique is introduced to improve spacer design. First, sufficient existing rheological property data (FANN readings) as a function of spacer density, temperature and polymer concentration are used to train the machine learning algorithm. Second, depending on the selected mud and cement, density of the spacer and the bottom-hole temperature, the required spacer properties are predicted by the trained machine learning algorithm for each polymer concentration. Third, given the wanted rheological properties, the best solution can be found by the inverse optimization algorithm with the least- square error in the range of polymer concentration. The proposed methodology is applied to a new class of spacer system, which is a premium, water-based spacer, designed to effectively displace the drilling fluid in the annulus. Hundreds of rheological properties are recorded with various combinations of density, temperature and polymer concentration. The data are used in training the nonlinear machine learning algorithm. In real cement-job design, with the required FANN readings of the spacer and FANN readings of mud and cement, the algorithm finds an optimized solution. The FANN readings predicted by the machine learning algorithm are compared in the inverse search step. The calculated polymer concentration and FANN readings are confirmed by laboratory tests with minor variations.
Acidizing of carbonate reservoirs is a common technique used to restore and enhance production by dissolving a small fraction of the rock to create highly conductive channels. Literature review reveals that most acidizing studies are focused on acid injection at a constant volumetric rate (CVR) instead of at a constant injection pressure (CIP). Therefore, the primary objective of the present work is to investigate the benefits and recommended applications of each technique. The study analyzes dissolution patterns, and wormhole propagation rate. A coreflood study was conducted using different Indiana limestone cores to assess both techniques. Additionally, a 2-D wormhole model was used to mathematically describe the acidizing phenomena. This model describes the reactive transport of acid as a coupling between Darcy scale flows. The algorithm captures the essential physics and chemistry of the acid reaction in a carbonate porous medium. The study confirmed that all types of acid dissolution patterns (i.e. face, conical, wormhole, and branched) exist for both techniques (CVR or CIP). Unlike in the CVR technique, dissolution patterns during the CIP technique can change, tending toward a branched dissolution regime. The CIP technique required a lower acid volume to achieve breakthrough in the conical dissolution regime and a higher acid volume to achieve breakthrough the branched dissolution regime compared to the CVR technique. In a dominant wormhole pattern, both techniques required nearly the same acid volume for breakthrough. A CT scan confirmed that the CIP technique develops a uniform wormhole at a low initial injection rate. For the CIP technique, the acid injection rate increased exponentially with the volume of the acid injected.
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