Summary Gel plugs (GP's) have been used in the eastern U.S. for many years. They are a versatile and simple method of temporarily plugging a well. Gel plugs are often used to control a well while remedial workovers are performed. This paper describes the various types of gel plugs and their applications. Case histories are given for gel plug applications.
Summary This paper introduces the pressure derivative in fracturing-pressure analysis. The derivative is shown to enhance the analysis capabilities significantly. The interpretation methodology is presented, and several field data sets and simulations are discussed to illustrate the technique.
Summary When fracture proppants are subjected to cyclic loading, the conductivity of the fracture is reduced. This paper presents laboratory results to quantify the degradation of sand and intermediate-strength proppants (ISP's) as closure stress is loaded cyclically to the proppant. The results imply that, if a deep gas well is opened and then shut in repeatedly, permanent damage to the hydraulic fracture will occur.
Summary This paper presents a finite-difference representation of the wave equation developed for diagnostic analyses of sucker-rod pumping systems. A consistent method of computing the viscous damping term associated with the damped-wave equation is also presented.
SummaryThe prediction of friction pressures for proppant-laden fracturing fluids requires estimations of both the base-gel friction factor and the effect of proppant on fluid rheology. This paper introduces two new expressions, each theoretically based with constants determined from data, that address these two issues for hydroxypropyl guar (HPG)-based fracturing fluids in laminar and turbulent flow.The paper first introduces a new expression for the turbulent friction factor of HPG base gels. This implicit expression for the friction factor is more theoretically correct and requires one less empirical constant than explicit forms currently used. The effect of proppant on the effective viscosity of non-Newtonian fluids is then discussed and a new expression, which includes shear rate, temperature, gel concentration, and proppant volume fraction as parameters, is derived. Developed from laboratory data and existing slurry rheology theories, this expression is shown to provide excellent predictions of laboratory and field data for both tubing and annulus injection.
Summary Application of water-soluble polymers in the oil industry (e.g., fluid-loss reducer, polymer flooding, and water-based drilling muds) requires hydrosoluble polymers to be compatible with corrodible materials. The behavior of polyacrylamides and xanthans in the presence of various materials used for oil production (steel, stainless steel, carbon steel, and Inconel™) has been studied vs. different water salinities, oxygen contents, and temperatures. The influence of such commonly used additives as oxygen scavengers and sequestrants on corrosion and polymer stability has also been investigated. For both types of polymers, as corrosion occurs under anaerobic conditions, strong interactions between polymer chains and divalent cations (Fe2+ or Ni2+) are observed. Such interactions also depend on polymer quality. In the presence of oxygen, corrosion induces a molecular-weight degradation of the polymer followed by a gelation process for xanthan. Some additives may accelerate the transformation of Fe2+ to Fe3+, thus inducing polymer degradation, but this reaction depends on the nature of the chelating agent. These results provide guidelines for the implementation of polymers in oil production, including the selection of materials, water treatment, or mud formulation.
SummarySixty-five percent of the reserves of the Kuparuk River field, the second-largest producing oil field in the U.S., is contained in a 20- to 80-md-permeability sandstone. This paper provides details of stimulation design advances made over the past 3 years in this formation. The design steps for optimizing fracture treatments in a moderate-permeability formation require primary emphasis on fracture conductivity rather than on treatment size or fracture length. This philosophy was used for the 140 new wells documented in this paper. Treatment size was gradually increased once a commensurate increase in fracture conductivity was obtained. Applying the new design to the refracturing of 88 producing wells in the field resulted in an incremental 40, 000 BOPD, a significant portion of the field's 300, 000 BOPD.
SummaryPseudo 3D (P3D) hydraulic fracturing models often overpredict fracture height for a poorly contained fracture. This is caused partly by either the neglect of the fluid flow component in the vertical direction or a crude treatment of the 2D fluid flow in the fracture as 1D flow in the vertical direction in the fracture-height calculation. This paper presents a height-growth model that adopts a flow field more representative of the actual 2D flow in a fracture. In this model, the fracture is divided into two regions: an inner region where the flow direction is nearly horizontal, and an outer region where the flow field is approximated by a radial flow from an imaginary source. The governing equations for determining height growth rate and the numerical method for solving these equations are described. A commercial P3D simulator was modified by replacing its original height-growth model with this 2D flow-height model. The modified simulator was tested against the original simulator and the Terra Tek and U. of Texas fully 3D simulators. The modified P3D simulator incorporating the new height model showed significant improvement over the original model in height calculations and good agreement with the fully 3D models.
Summary Minimizing or removing formation damage is a major objective in completion and stimulation operations. Formation damage is minimized by selection of "nondamaging" fluid systems and operations on the basis of petrographic analysis and field experience. A technique known as the capillary suction time (CST) test, adopted from the drilling-fluid industry, provides a quick and inexpensive method for qualitative selection or screening of the least damaging fluid system for a particular formation. The CST test is a fundamental filtration method for determining the electrolyte concentration that will produce the maximum inhibiting effect on a formation. The method uses an instrument to measure the time required for a liquid to travel a calibrated distance on a standard porous filter paper. The CST procedure currently is used very successfully to select completion fluids and to evaluate stimulation-fluid additives. This paper explains the procedure, discusses limitations of the test, and presents laboratory and field data to show the value of the technique.
Summary Although acidization has been used successfully for many years to increase the productivity of petroleum wells in carbonate formations, demands on the performance and application of the acidizing process are increasing. This study investigated a method of in-situ foam generation that allows deeper wormhole penetration yet uses less acid than conventional methods. The dissolution patterns were imaged with neutron radiography, which provided an in-depth understanding of the effects of foam and other critical parameters. Results show that foam is effective in promoting efficient stimulation, even at low acid injection rates.
SummaryThis paper describes a theoretical approach to determine the inhibitor dynamic adsorption isotherm from coreflood experiments. The main feature of the isotherm that contributes principally to the long squeeze life is highlighted. The problems of modeling near-well squeeze treatments and an improved simulator are discussed.
Summary This paper presents a reliable model for the solubility products of scale-forming minerals. Our model solubilities are in relatively good agreement (±5% to 10%) with the most reliable solubilities in aqueous solutions of up to twice the seawater concentration at temperatures of 20 to 150°C and pressures up to 40 MPa. An equilibrium model for the reactions responsible for scale formation is developed. The CO2 equilibria between the gas, oil, and water phases and in the water phase itself are considered. By combining a hydrodynamic model for the transport of water through an oil reservoir during waterflooding with the proposed equilibrium model, we can estimate the amount of precipitate formed in the reservoir.
Summary Coiled-tubing cementing has been practiced successfully on the Alaskan North Slope for several years. This paper discusses the special problems faced when this technology was applied to offshore U.S. gulf coast operations. The innovative solutions and procedures developed to improve the economic and technical success of coiled-tubing cementing are also discussed. Comparative laboratory and computer studies, as well as field case histories, will be presented to show the economic merit of this technology.
Summary This paper describes a new formulation of nonlinear soil mechanics and multiphase thermal flow. The nonlinearities of the soil behavior and their interactions with fluid flow causing shear failure of the soil are the dominant features of the process. The numerical formulation of the coupled flow/stress solution model includes nonlinear compressibility and flow properties as functions of pressure, stress, and temperature; nonlinear, incremental, thermal poroelastic stress analysis; and shear or tensile failure and its effects on transport properties, porosity, and stress. An efficient sequential numerical scheme was developed. It is mass conservative and applicable to external coupling of existing simulators. The 1D examples show some startling new features of reservoir mechanics in unconsolidated media.
SummaryOilfield produced water containing a high concentration of total dissolved solids (TDS) and hardness can successfully be softened for use as oilfield steam-generator feedwater. At the Belridge field in Kern County, CA, the combination of caustic softening and weak-acid cation exchange has been used to soften produced water containing 11, 000 TDS and 550-ppm hardness to < 1-ppm hardness. The resultant sludge containing calcium carbonate and magnesium hydroxide is concentrated by centrifuging and is disposed of in a landfill. Compared to the use of conventional strong-acid ion exchange followed by weak acid or weak acid followed by weak-acid ion exchange systems, the process offers the benefits of lower capital and chemical costs, partial silica removal, and elimination of liquid waste discharge. This paper gives design parameters and operating conditions and discusses future applications in thermal recovery projects.
SummaryTwo integral equations are derived for the 3D problem of a pressurized plane crack in an infinite, fluid-saturated, poroelastic solid. These equations relate normal tractions and fluid pressures on the crack faces to crack openings and fluid injection rate per unit fracture area. An important application of these equations is the prediction of hydraulic fractures induced during waterflooding of reservoirs to enhance gas and oil recovery.
Summary Mathematical modeling and test data were used to predict the temperatures of equipment in API fire testing. The thermal response of these systems is used to select critical parts for fire-resistant equipment. Conditions of internal components were found to be calculable on the basis of defined test criteria.
Summary This paper investigates the effect of electric submersible pump (ESP) performance tolerances and minor speed variations on the producing rate of wells completed in underpressured reservoirs, and presents ESP design considerations unique for this class of wells. These wells require considerable head to initiate flow and have relatively flat well-load curves. Pumps that operate near their maximum recommended rate have steep performance curves, and this is shown to minimize the effect of an underperforming pump on producing rate. Equations are developed for calculating the effects of pump performance and speed. Application requires evaluating the slopes of the pump-performance and well-load curves at design rate. The usefulness of these equations is demonstrated by practical examples. It is also demonstrated that flow stall can occur easily in underpressured reservoir applications when pumps designed to operate near their minimum recommended rate are installed.
Summary The sour-gas wells in the Big Escambia Creek (BEC) field of South Alabama have a production environment that consists of 280°F, 21% H2S, 40% CO2, and up to 190, 000 ppm chlorides in the produced water. The highly corrosive conditions demand the ultimate in a corrosion mitigation program to produce these wells safely and economically. This paper describes the background, technical development, and results of the downhole and gathering-system corrosion inhibition and monitoring programs: continuous downhole inhibition by means of annular injection of a water-dispersible inhibitor in the most aggressive wells; downhole batch treatments with nitrified tubing displacements of an oil-soluble inhibitor in the remaining wells; continuous injection of a water-dispersible inhibitor in the gathering lines to augment the batch treatments and in selected highly corrosive wells; monitoring of inhibitor residuals and plotting of trends to ensure the effectiveness of downhole treatments and to optimize injection rates; and flowline calipers and hydrotests to monitor the gathering-line inhibition programs. The paper also describes the computerized approach used in designing, calculating, and updating both the continuous downhole injection system and the nitrified batch treatments. The success of the inhibition and monitoring program is demonstrated by the tubing life achieved, field data on inhibitor residuals, and flowline and downhole calipers. The technical data presented will aid in the design and implementation of successful inhibition programs for highly corrosive production environments.
SummaryConstant-height models, in the absence of significant barrier stresses, frequently overpredict fracture half-lengths. This has been a problem in the fracture design of the Frontier and Dakota formations in the Greater Green River basin. Both formations often experience unlimited fracture-height growth as a result of a lack of barrier-stress contrast. This paper presents some of the implications of uncontained fracture-height growth in these formations on fracture design through 2D and 3D fracture geometry modeling. The effects of the minimum in-situ-stress profile, Young's modulus profile, and fluid-loss contrast on the fracture geometry evolution are studied. Actual field example data from a Frontier well are used for the sensitivity analyses.
