Diederik van Batenburg

Fracture-Related Diagenesis May Impact Conductivity

Rapid loss of fracture conductivity after hydraulic fracture stimulation has often been attributed to the migration of formation fines into the proppant pack or the generation of fines derived from proppant crushing. Generation of crystalline and amorphous porosity-filling minerals can occur within the proppant pack because of chemical compositional differences between the proppant and the formation, and the compaction of the proppant bed because of proppant pressure solution reactions. Findings presented in this paper suggest that diagenesis-type reactions that can occur between proppant and freshly fractured rock surfaces can lead to rapid loss of proppant-pack porosity and loss of conductivity.

Flexible Spectral Methods for the Generation of Random Fields with Power-Law Semivariograms ~

Random field generators serve as a tool to model heterogeneous media for applications in hydrocarbon recovery and groundwater flow. Random fields with a power-law variogram structure, also termed fractional Brownian motion (fBm) fields, are of interest to study scale dependent heterogeneity effects on one-phase and two-phase flow. We show that such fields generated by the spectral method and the Inverse Fast Fourier Transform (IFFT) have an incorrect variogram structure and variance. To illustrate this we derive the prefactor of the fBm spectral density function, which is required to generate the fBm fields. We propose a new method to generate fBm fields that introduces weighting functions into the spectral method. It leads to a flexible and efficient algorithm. The flexibility permits an optimal choice of summation points (that is points in frequency space at which the weighting function is calculated) specific for the autocovariance structure of the field. As an illustration of the method, comparisons between estimated and expected statistics of fields with an exponential variogram and of fBm fields are presented. For power-law semivariograms, the proposed spectral method with a cylindrical distribution of the summation points gives optimal results.

A comparison of freshwater- and seawater-based borate-crosslinked fracturing fluids

This paper discusses the chemical factors that operators must address to successfully substitute seawater for fresh water in borate-crosslinked guar fracturing fluids. Seawater contains cations and anions that affect the performance of the fracturing fluids components, as well as the fluids interaction with the formation. Because seawater has high ionic strength, it lowers the viscosity obtained from borate-crosslinked guar. High magnesium in the water consumes hydroxide ions and affects pH control, which in turn affects the equilibrium borate-ion concentration. This paper addresses these problems and provides guidelines for borate fluid formulation to offset seawater characteristics for temperatures as high as 300°F. Borate fluids from seawater can control fluid loss (FL) as well as freshwater borate fluids. Chemical gel breakers for offshore environments have been developed to help control the viscosity reductions in fracturing fluids. Conductivity values with seawater fluids are equal to or better than freshwater fluids. The presence of divalent cations in the seawater fluid caused no harm to the FL properties or the retained conductivity with borate seawater fluids.

Understanding proppant flowback

Proppant production from hydraulically fractured wells can cause severe operational problems, increase safety concerns, and dramatically reduce economic returns on well-stimulation investments. Methods that have helped eliminate or minimize proppant flowback include modified completion designs, the use of controlled fracture closure for obtaining early closure on the proppant pack, and the use of materials designed to reduce proppant production. Curable resin-coated proppants, chopped fiberglass. thermoplastic strips, and chemicals that modify the surface of the proppant are all accepted methods for minimizing flowback This paper presents the results of both physical and numerical modeling of proppant flowback recorded during the development of a chemical designed for modifying the proppant surface. The goal of this study was to develop an understanding of the mecha nisms that control proppant flowback. Laboratory experiments performed in slot models with no closure stress helped establish the interaction of proppant size, proppant distribution, and fluid velocity. Additional studies of the impact of closure stress, fracture width, and fluid rate on proppant flowback were performed with modified API linear conductivity cells. Data obtained from the physical modeling were used to calibrate a numerical model that predicts proppant flowback. In this model, fluid flow in the proppant pack is described by Darcys equation for flow through porous media. The resulting velocity distribution allowed local stability to be assessed along the free surface between the proppant pack and the continuous fluid phase. Repeating these steps allowed evaluation of the interface that develops over time.

Acoustic Stimulation to Mitigate Near-Wellbore Damage

Near-wellbore formation damage reduces productivity in oil and gas wells. Formation damage can be caused by fluid invasion during the drilling and completion process and by particles in the production stream. The conventional technique to remove this damage is fluid injection to dissolve the damage. A promising new technique to remove the near-wellbore damage is high-frequency acoustic stimulation. A specially designed acoustic tool is deployed on wireline or coiled tubing (CT) to selectively treat damaged reservoir intervals.

Near Wellbore Stimulation by Acoustic Waves

Reduced well productivity or injectivity is often caused by near-wellbore formation damage due to the interaction of reservoir formation with drilling and completion fluids. The problem can be further compounded by production induced formation damage. A prime example is fines migration and fines plugging of rock pores or gravel packs. High frequency sonic waves, especially ultrasonic waves have been used in many industrial applications to remove contaminants like dirt, oil, and grease from parts immersed in fluids. An obvious extension of this application is the removal of wellbore impairment by exposing it to high frequency acoustic waves. Although the concept is old, successful largescale application of acoustic well stimulation is not common. Greater understanding of the technology’s applicability and limitations are essential in order to design effective downhole acoustic tool and guide successful field implementation. To this end, we have embarked on a dedicated project to mature the technology, which includes dedicated experimentation and tool design. In this paper, we focus on some key experimental results and discuss potential applications in production engineering.

Screening considerations for curable resin-coated proppants

Curable resin-coated proppants (RCP) have been used to minimize proppant flowback from propped hydraulic fractured oil and gas wells for years, yet proppant backproduction is still reported to he a major operational problem. The industry cannot currently explain why some hydraulic fractures propped with curable RCP produce proppant back while others containing only uncoated proppant do not produce any proppant back. This paper presents results ofa study done to help determine screening considerations for curable resin-coated proppants. The study involved curing and crushing proppant plugs under a variety of conditions ranging from frac-and-pack applications in shallow wells to fracturing applications in deep horizontal wells. These tests were conducted to identify suitable RCPs for given applications. The unconfined compressive strength (UCS) of proppant plugs cured in the treatment fluid at reservoir temperature with closure stress applied. is currently used as the screening criterion for curable RCP. The UCS needs to be as high as possible while the same resin coating should not consolidate the proppant while left in the well after a treatment without closure stress applied. This study demonstrates that at least six data points are needed to obtain a representative value for the unconfined compressive strength (UCS) of proppant for any one set of conditions. Although several studies of factors affecting the performance of RCPs have been presented in the literature. the conclusions in these studies are generally based on limited data for one set of conditions and are. therefore. often questionable. In addition, the effect of cooldown and subsequent heatup on RCPs after a treatment has not been considered previously as a factor in proppant backproduction. This paper demonstrates that this temperature effect is an important parameter to consider. Stress cycling has been identified as one of the failure mechanisms for RCP in experiments. These experiments were conducted at ambient temperature with proppant packs cured at temperatures between 160 and 200°F (71 and 93°C). We conducted similar experiments at simulated in-situ conditions to verify whether the mechanism was still effective for these conditions. We found that the addition of thermoplastic film material to RCP cured at 300°F (149°C) increased its resistance to stress cycling.

Production Gains from Re-Fracturing Treatments in Hassi Messaoud, Algeria

Since 1990, over 200 hydraulic fracturing treatments have been performed in the Hassi Messaoud field in northeast central Algeria resulting in an average production increase of 5 m 3 /hr (750 bopd). The success of the stimulation program in this Cambrian age sandstone formation resulted from improved field practices of treating open-hole and slotted liner completions, and from the use of state-of-the-art fracturing equipment and engineering tools. After several years of production, the production rates of some fractured wells have declined due to several reasons: reservoir depletion, salt precipitation from formation water breakthrough or injection water breakthrough, and fracture conductivity damage due to barium sulfate scale and asphaltene deposition. Since 1996, re-fracturing treatments have been conducted on nine wells in order to restore production by placing another propped hydraulic fracture in the same target drain. The three case histories presented show exceptional, normal and marginal responses to the re-fracturing treatments. Explanations for the observed responses are presented and recommendations for the selection of new re-fracturing candidates in the Hassi Messaoud field are presented.

New Techniques for Hydraulic Fracturing in the Hassi Messaoud Field

The Hassi Messaoud oilfield is a thick sandstone reservoir in the north-eastern part of central Algeria. The reservoir was discovered in 1956 and produces from a Cambrian-age sandstone at approximately 3400 meters depth. Hydraulic fracturing has been successfully used to improve oil production since 1990. Pre-treatment injection tests have been conducted on approximately 250 wells and proppant has actually been placed in approximately 200 wells. The 50 wells on which only injection tests were conducted were reviewed. The review showed that there were three main reasons why treatments were aborted after the injection tests: (1) mechanical failure of the well, (2) treating pressure too high, and (3) fracture geometry, as interpreted from temperature logs, created outside target interval. In order to increase the success rate of the treatments two new techniques were recently introduced. ○ Propellant stimulation techniques were used to reduce fracture initiation pressures and to control the fracture initiation point. ○ A dual fracturing technique was introduced to control downward growth of the created fractures. The paper presents three well-documented case histories to illustrate the application of these new techniques. The data presented includes pre- and post-treatment production, peak pressures from the propellant runs, temperature and radioactive tracer logs from after the injection tests, and radioactive tracer logs that show the actual placement of the proppant.

Near-Wellbore Temperature Simulations Assist Design of Water-Shutoff Treatments

Until recently, most water-shutoff treatments were designed for wells needing low-volume treatments. However, many highly productive reservoirs are beginning to produce high volumes of water. The evolution of diagnostic and interpretive techniques such as reservoir description and modeling and production logging have significantly enhanced the degree of accuracy and completeness of production problem diagnoses. Many of these tools can also be used for more effective water-shutoff treatment designs. Near-wellbore temperature simulations allow treatment designs based on realistic treatment temperatures, rather than on bottomhole static temperatures. This paper presents a method of conducting temperature simulations that is applicable to all water-shutoff treatments. This paper describes how engineers used temperature simulation results to design a water-shutoff treatment and its placement for a well in the Norwegian Sector of the North Sea. The results of the simulations allowed optimized treatment placement rates, fluid composition, and shut-in times of the job and constructed temperature histories for different stages of the treatment. These histories clearly showed that different activator compositions and/ or concentrations were required for early, intermediate, and final treatment stages. The results of near-wellbore temperature simulations allowed engineers to predict the effects of the following factors: . interval permeability distribution . treatment rate . viscosities of injected fluids.