The Effects of Initial Condition of Fracture Surfaces, Acid Spending, and Type on Conductivity of Acid Fracture PDF Download

Author: Ali Mansour A. Almomen
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Languages : en
Pages : 92

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Fracture conductivity and the effects of treatment variables can be studied in the laboratory. We conducted experiments based on scaling down the field conditions to laboratory scale by matching Reynold's and Peclet numbers. Experiments conducted were comprised of three stages: dynamic etching, surface characterization of etched cores, and conductivity measurement. The effect of initial condition of fracture surfaces on the etching pattern and conductivity were investigated in this study. Another area of interest is the variation of conductivity along the fracture due to acid spending. We also investigated the contact time, acid system type, and treatment temperature effects on conductivity using San Andres dolomite cores. The results from these studies showed that rough-surface fractures generate higher conductivity by an order of magnitude compared with a smooth-surface fracture at low-closure stress. Also, conductivity generated on rough-surface fractures by smoothing peaks and deepening valleys which widen the gap between the fracture surfaces after closure and acid creates conductivity on smooth-surface fractures by differential etching that creates asperities. The results suggest that an increase in acid spending does not automatically result in lower conductivity; and etched volume alone is not adequate to predicate the conductivity. Conductivity results from a combination of etching pattern, etched volume, and rock compressive strength after etching. In-situ crosslinked acid was found to be more effective in etching rock and controlling acid leakoff compared with linear-gelled acid. Also, crosslinked acid reduces the number of pits and the pit diameters. Based on conductivity tests, linear-gelled acid is more favorable at higher temperatures while in-situ crosslinked acid showed higher conductivity at lower temperatures. For a rough-surface fracture, shorter contact time created high conductivity compared to longer contact while injecting the same volume of acid, suggesting the existence of an optimum contact time. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151186

Author: Andrea Nino Penaloza
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Languages : en
Pages : 90

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Acid fracture conductivity and the effect of key variables in the etching process during acid fracturing can be assessed at the laboratory scale. This is accomplished by using an experimental apparatus that simulates acid injection fluxes comparable to those in actual acid fracture treatments. After acid etching, fracture conductivity is measured at different closure stresses. This research work presents a systematic study to investigate the effect of temperature, rock-acid contact time and initial condition of the fracture surfaces on acid fracture conductivity in the Austin Chalk formation. While temperature and rock-acid contact are variables normally studied in fracture conductivity tests, the effect of the initial condition of the fracture surface has not been extensively investigated. The experimental results showed that there is no significant difference in acid fracture conductivity at high closure stress using smooth or rough fracture surfaces. In addition, we analyzed the mechanisms of acid etching and resulting conductivity creation in the two types of fracture surfaces studied by using surface profiles. For smooth surfaces, the mechanism of conductivity creation seems connected to uneven etching of the rock and roughness generation. For rough surfaces, acid conductivity is related to smoothing and deepening of the initial features on the sample surface than by creating more roughness. Finally, we compared the experimental results with Nirode-Kruk correlation for acid fracture conductivity. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149578

Author: Maria Georgina Melendez Castillo
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Languages : en
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The conductivity created in acid fracturing is a competition between two phenomena: etching of the rock surface and weakening of the rock. This study presents experimental results of acid fracturing conductivity experiments with polymer gelled acid, while varying contact time and rock type. The experiments were conducted in a laboratory facility properly scaled from field to laboratory conditions to account for the hydrodynamic effects that take place in the field. The rocks of study were Indiana limestone, San Andres dolomite and Texas Cream chalk. Our results illustrate that acid fracturing conductivity is governed by the etching pattern of the rock surface and influenced by the hardness of the rock. If channels are created, the fracture is more likely to retain conductivity after closure. The hardness of the rock is the dominating factor to determine the conductivity response when no channeling is present. Among the rocks tested, Texas Cream chalk had the lowest hardness measurement before and after acidizing and the fracture closed at a much lower stress compared with limestone and dolomite. Dolomite had the highest conductivity under all closure stresses even without a channeling pattern. Additionally, it was observed that a higher reduction in rock strength at the contact points for dolomite yielded lower conductivity after closure. The effects of hardness variation on conductivity are higher in dolomite than in limestone and chalk. It is apparent that longer contact times do not always provide higher conductivity after closure.

Author: Jarrod Thomas Underwood
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Languages : en
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Acid fracturing is a well stimulation strategy designed to increase the productivity of a producing well. The parameters of acid fracturing and the effects of acid interaction on specific rock samples can be studied experimentally. Acid injection data and fracture conductivity measurements obtained in the research presented in this thesis yielded results that qualified and quantified the impact of a specific acid system on rock samples of varying acid solubility. Six rock samples from a carbonate reservoir were labeled A through F to protect proprietary information included in this research. A 2% potassium chloride solution was used for the acid system and fracture conductivity measurements to prevent clay swelling. Injection temperature, contact time, and injection rate were designed to simulate field treatment conditions. The effects of a chelating agent on fracture conductivity were also studied. Before and after images of the rock samples indicated that the effect of 15% hydrochloric acid on the samples was limited but correlated with the rock acid solubility. Samples E and F had a greater value of acid solubility and showed noticeable surface etching. Samples A, B, and C had lower values of acid solubility and did not show signs of surface etching. Sample D was of moderate acid solubility and showed minimal signs of surface etching. Fracture conductivity did not correlate directly with acid solubility, but likely was a function of inherent matrix permeability based on leak-off measurements and fracture conductivity measurements. Finally, the fracture conductivity of Sample D increased after exposure to a chelating agent. Commonly, acid fracture experimental studies are carried out with outcrop rock samples. The samples have more homogenous properties and without hydrocarbon content. In this study, cores from downhole formation were used. The original condition was preserved as much as possible to simulate real field situations. However, using field rock samples does present challenges not generally associated with outcrop rock samples. Based on the information gathered from the work presented in this thesis, conclusions were drawn concerning the effectiveness of a 15% hydrochloric acid treatment in this formation and the challenges of using field rock samples. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151691

Author: Maysam Pournik
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Languages : en
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Success of acid fracturing treatment depends greatly on the created conductivity under closure stress. In order to have sufficient conductivity, the fracture face must be non-uniformly etched while the fracture strength maintained to withstand the closure stress. While there have been several experimental studies conducted on acid fracturing, most of these have not scaled experiments to field conditions and did not account for the effect of rock weakening and etching pattern. Hence, acid fracture conductivity predictions based on the above works have not been able to match actual results. In order to develop a more appropriate and accurate prediction of acid fracturing treatment outcome, a laboratory facility was developed that is properly scaled to field conditions and enables analysis of etching pattern and rock strength. A systematic experimental study that covered a variety of formations, acid types, and acid contact times was conducted. An acid fracture conductivity correlation was developed based on etched volume, etched pattern, and fracture strength under closure stress. Results suggested that there is an optimal time of acid exposure resulting in maximum fracture conductivity. There were large differences in the conductivity created with the different acid systems tested due to different etching patterns and degree of rock strength weakening. There was an optimal acid system depending on formation type, contact time and overburden stress. The acid fracture conductivities measured did not agree with the predictions of the Nierode-Kruk correlation. The newly developed correlation predicts conductivity much closer as it includes the effect of rock strength and surface etching pattern on resulting conductivity.

Author: Abdulwahab Alghamdi
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Languages : en
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Acid fracturing is one of the preferred methods to stimulate wells in carbonate reservoirs. It consists of injecting an acid solution at high enough pressure to break down the formation and to propagate a two-wing crack away from the wellbore. The acid reacts with the carbonate formation and this causes the etching of the fracture surfaces. After the treatment, the created etched surfaces do not close perfectly and thatl eaves behind a highly conductive path for the hydrocarbons to be produced. We distinguish the issue of treatment sizing (that is the determination of the volume of acid to be injected) and the issue of creating optimum fracture dimensions given the size of the treatment. This is reasonable because the final cost of a treatment is determined mainly by the volume of acid injected and our goal should be to achieve the best performance of the treated well. The well performance depends on the created fracture dimensions and fracture conductivity and might change with time due to various reasons. This research evaluates two field cases from Saudi Aramco where acid fracturing treatment has been used to stimulate a carbonate formation. I investigated the following issues: a) how effective was the treatment to restoring the initial productivity, b) how did the productivity of the well change with time; c) what are the possible reasons for the change in performance, d) what are our options to improve acid fracture design in the future?Based on our research work both near-well liquid drop-out and fracture-conductivitydeterioration can impact the production in different proportion. Moreover, the fracturing model tends to overestimate the fracture conductivity in some cases as shown in SA-2. Also, the "Acid fracture Number" concept proves to be an effective way to evaluate the acid fracturing treatment. Several recommendations were made based on this research work as described in the last part of my thesis.

Author: Jianye Mou
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Languages : en
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Success of acid fracturing depends on uneven etching along the fracture surfaces caused by heterogeneities such as variations in local mineralogy and variations in leakoff behavior. The heterogeneities tend to create channeling characteristics, which provide lasting conductivity after fracture closure, and occur on a scale that is neither used in laboratory measurements of acid fracture conductivity, which use core samples that are too small to observe such a feature, nor in typical acid fracture simulations in which the grid block size is much larger than the scale of local heterogeneities. Acid fracture conductivity depends on fracture surface etching patterns. Existing acid fracture conductivity correlations are for random asperity distributions and do not consider the contribution of channels to the conductivity. An acid fracture conductivity correlation needs the average fracture width at zero closure stress. Existing correlations calculate average fracture width using dissolved rock equivalent width without considering the effect of reservoir characteristics. The purpose of this work is to develop an intermediate-scale acid fracture model with grid size small enough and the whole dimension big enough to capture local and macro heterogeneity effects and channeling characteristics in acid fracturing. The model predicts pressure field, flow field, acid concentration profiles, and fracture surface profiles as a function of acid contact time. By extensive numerical experiments with the model, we develop correlations of fracture conductivity and average fracture width at zero closure stress as a function of statistical parameters of permeability and mineralogy distributions. With the model, we analyzed the relationships among fracture surface etching patterns, conductivities, and the distributions of permeability and mineralogy. From result analysis, we found that a fracture with channels extending from the inlet to the outlet of the fracture has a high conductivity because fluid flow in deep channels needs a very small pressure drop. Such long and highly conductive channels can be created by acids if the formation has heterogeneities in either permeability or mineralogy, or both, with high correlation length in the direction of the fracture, which is the case in laminated formations.

Author:
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Category : Weights and measures
Languages : en
Pages : 476

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Author: Jiayao Deng
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Languages : en
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The conductivity of acid-etched fractures highly depends on spaces along the fracture created by uneven etching of the fracture walls remaining open after fracture closure. Formation heterogeneities such as variations of mineralogy and permeability result in channels that contribute significantly to the fracture conductivity. Current numerical simulators or empirical correlations do not account for this channeling characteristic because of the scale limitations. The purpose of this study is to develop new correlations for conductivity of acid-etched fracturing at the intermediate scale. The new correlations close the gap between laboratory scale measurements and macro scale acid fracture models. Beginning with acid-etched fracture width profiles and conductivity at zero closure stress obtained by the previous work, I modeled the deformation of the fracture surfaces as closure stress is applied to the fracture. At any cross-section along the fracture, I approximated the fracture shape as being a series of elliptical openings. With the assumption of elastic behavior for the rock, the numerical simulation presents how many elliptical openings remain open and their sizes as a function of the applied stress. The sections of the fracture that are closed are assigned a conductivity because of small-scale roughness features using a correlation obtained from laboratory measurements of acid fracture conductivity as a function of closure stress. The overall conductivity of the fracture is then obtained by numerically modeling the flow through this heterogeneous system. The statistical parameters of permeability distribution and the mineralogy distribution, and Young's modulus are the primary aspects that affect the overall conductivity in acid-etched fracturing. A large number of deep, narrow channels through the entire fracture leads to high conductivity when the rock is strong enough to resist closure stress effectively. Based on extensive numerical experiments, I developed the new correlations in three categories to predict the fracture conductivity after closure. Essentially, they are the exponential functions that incorporate the influential parameters. Combined with the correlations for conductivity at zero closure stress from previous work, the new correlations are applicable to a wide range of situations.

Author: Cassandra Vonne Oeth
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Languages : en
Pages :

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Acid fracture stimulation generates higher well production but requires engineering design for treatment optimization. To quantify the cost and benefit of a particular acid fracture treatment an engineer must predict the resulting fracture's conductivity, which is based on the etched width created by the injected acid. Etching occurs along the fracture surface but is based on acid flowing through the fracture, so an evaluation tool should describe three-dimensional physics and chemistry. Current practice is to estimate conductivity utilizing two-dimensional models. Unfortunately, these models necessarily assume how acid is distributed in the fracture and often misrepresent the amount of acid etching upon which the conductivity is based. A fully three-dimensional modeling tool to evaluate and predict acid fracture performance across the wide range of carbonate field properties has been developed. The model simulates acid transport and fracture face dissolution. The acid transport model includes the solution of the three-dimensional velocity and pressure fields, the non-Newtonian characteristics of most acid fracturing fluids, and diffusion of acid toward the fracture surface. The model numerically solves the equations describing the three-dimensional acid transport and reaction within a fracture to yield the etched width created by acid along the fracture. The conductivity is calculated with the simulator derived acid-etched width, using correlations recently developed that reflect the small scale heterogeneity of carbonate rock as it creates etching along the fracture surface. The performance of an acid fracture treatment is quantified with conductivity, which is strongly dependent on the etched width created by the acid. This robust new tool more accurately models the impact of design decisions on the acid-etched width and provides a rational path for treatment optimization. Cases typical of industry practice are presented that demonstrate the model capabilities. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151892