Advanced Fracturing Design Simulator-Assisted Modeling Coupled with Application of Enhanced Stimulation Fluids Raises Performance of Acid Fractured Wells

Abstract Important factors affecting acid fracturing efficiency include etched fracture geometry, cleanup, and optimum differential etching to retain open channels after fracture closure. A recently applied integrated approach combined improvements in all three factors: new fracture simulation techniques enabled fracture geometry optimization, single-phase retarded acid provided significant increase in half-length, and high retained permeability viscous fluids supported better fracture cleanup. The approach was successfully implemented in several carbonate oil fields and resulted in a substantial productivity index increase. The approach enables acid fracture optimization in three steps. First, the high retained permeability, low-pH pad fluids and polymer-free leakoff control acids are used in combination to enhance formation cleanup after a treatment and to reduce the concentration of polymers in fissures network of naturally fractured carbonate reservoirs. Second, a new single-phase retarded acid is used to achieve longer half-length due to retarded reaction with formation rock and favorable viscous fingering effects. Third, a new acid fracturing simulation model is used to optimize fracture geometry. The simulation technique employs an innovative transport model that includes the viscous fingering effect, advanced leakoff simulation, changing acid rheology upon spending, and a novel calculation approach to mixed fluids' rheology. This combined concept was applied during acid fracturing treatments in moderate permeability wells of carbonate reservoirs with target intervals up to 4,600 m TVD and temperatures up to 125°C. The treatments consisted of guar-free low-pH pad fluid, polymer-free leakoff control acid, and single-phase retarded acid. Treatment optimization was performed using an advanced acid fracturing simulator to properly address the transport processes within the fracture in a low-stress-contrast environment. After the treatments, the pressure transient analysis indicated a strong linear regime for more than 15 hours, indicating effective fracture half-length at least 25% higher than average half-length after acid fracturing in offset wells where the conventional approach had been applied. Post-treatment half-length calculations showed a good match with advanced simulator results and proved the importance of accounting for viscous fingering effects during acid fracture half-length calculations. Calculation of the productivity index from the production data showed at least 15% increase compared to conventional acid fracturing treatments. The post-fracturing production decline rate was at least 20% slower than that of the conventional treatment in offset wells, which can be explained by the longer conductive fracture.

Download Full-text

Modeling Acid Fracturing Treatments with Multi-Stage Alternating Injection of Pad and Acid Fluids

10.2118/203985-ms ◽

2021 ◽

Author(s):

Rencheng Dong ◽

Mary F. Wheeler ◽

Hang Su ◽

Kang Ma

Keyword(s):

Viscous Fingering ◽

High Viscosity ◽

Carbonate Reservoirs ◽

Acid Etching ◽

Fracture Geometry ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Fracture Surfaces ◽

Low Viscosity ◽

Multi Stage

Abstract Acid fracturing technique is widely applied to stimulate the productivity of carbonate reservoirs. The acid-fracture conductivity is created by non-uniform acid etching on fracture surfaces. Heterogeneous mineral distribution of carbonate reservoirs can lead to non-uniform acid etching during acid fracturing treatments. In addition, the non-uniform acid etching can be enhanced by the viscous fingering mechanism. For low-perm carbonate reservoirs, by multi-stage alternating injection of a low-viscosity acid and a high-viscosity polymer pad fluid during acid fracturing, the acid tends to form viscous fingers and etch fracture surfaces non-uniformly. To accurately predict the acid-fracture conductivity, this paper developed a 3D acid fracturing model to compute the rough acid fracture geometry induced by multi-stage alternating injection of pad and acid fluids. Based on the developed numerical simulator, we investigated the effects of viscous fingering, perforation design and stage period on the acid etching process. Compared with single-stage acid injection, multi-stage alternating injection of pad and acid fluids leads to narrower and longer acid-etched channels.

Download Full-text

Modeling Multistage Acid Fracturing Treatments in Carbonate Reservoirs

10.2118/204197-ms ◽

2021 ◽

Author(s):

Rencheng Dong ◽

Mary F. Wheeler ◽

Hang Su ◽

Kang Ma

Keyword(s):

Radial Flow ◽

Viscous Fingering ◽

Carbonate Reservoirs ◽

Etching Process ◽

Acid Etching ◽

Fluid Viscosity ◽

Vertical Wells ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Carbonate Formation

Abstract As our industry is tapping into tighter carbonate reservoirs than in the past, completion techniques need to be improved to stimulate the low-permeability carbonate formation. Multistage acid fracturing technique has been developed in recent years and proved to be successful in some carbonate reservoirs. A multistage acid fracturing job is to perform several stages of acid fracturing along a horizontal well. The goal of acid fracturing operations is to create enough fracture roughness through differential acid etching on fracture walls such that the acid fracture can keep open and sustain a high enough acid fracture conductivity under the closure stress. In multistage acid fracturing treatments, acid flow is in a radial flow scenario and the acid etching process can be different from acid fracturing in vertical wells. In order to accurately predict the acid-fracture conductivity, a detailed description of the rough acid-fracture surfaces is required. In this paper, we developed a 3D acid transport model to compute the geometry of acid fracture for multistage acid fracturing treatments. The developed model couples the acid fluid flow, reactive transport and rock dissolution in the fracture. We also included acid viscous fingering in our model since viscous fingering mechanism is commonly applied in multistage acid fracturing to achieve non-uniform acid etching. Our simulation results reproduced the acid viscous fingering phenomenon observed from experiments in the literature. During the process of acid viscous fingering, high-conductivity channels developed in the fingering regions. We modeled the acid etching process in multistage acid fracturing treatments and compared it with acid fracturing treatments in vertical wells. We found that due to the radial flow effect, it is more difficult to achieve non-uniform acid etching in multistage acid fracturing treatments than in vertical wells. We investigated the effects of perforation design and pad fluid viscosity on multistage acid fracturing treatments. We need to have an adequate number of perforations in order to develop non-uniform acid etching. We found that a higher viscosity pad fluid helps acid to penetrate deeper in the fracture and result in a longer and narrower etched channel.

Download Full-text

Modeling Acid Fracturing Treatments in Heterogeneous Carbonate Reservoirs

10.2118/204304-ms ◽

2021 ◽

Author(s):

Rencheng Dong ◽

Mary F. Wheeler ◽

Hang Su ◽

Kang Ma

Keyword(s):

Reactive Transport ◽

Transport Model ◽

Viscous Fingering ◽

Carbonate Reservoirs ◽

Acid Etching ◽

Fracture Conductivity ◽

Acid Fracturing ◽

Fracture Surfaces ◽

Etching Pattern ◽

Viscous Fingers

Abstract The goal of acid fracturing operations is to create enough fracture roughness through non-uniform acid etching on fracture surfaces such that the acid fracture can keep open and sustain a high enough acid fracture conductivity under the formation closure stress. A detailed description of the rough acid-fracture surfaces is required for accurately predicting the acid-fracture conductivity. In this paper, a 3D acid transport model was developed to compute the geometry of acid fracture for acid fracturing treatments. The developed model couples the acid fluid flow, reactive transport and rock dissolution in the fracture. We also included acid viscous fingering in our model since the viscous fingering mechanism is commonly applied in acid fracturing to achieve non-uniform acid etching. Carbonate reservoirs mainly consists of calcite and dolomite minerals but the mineral distribution can be quite heterogeneous. Based on the developed model, we analyzed the effect of mineral heterogeneity on the acid etching process. We compared the acid etching patterns in different carbonate reservoirs with different spatial distributions of calcite and dolomite minerals. We found that thin acid-etched channels can form in carbonate reservoirs with interbedded dolomite layers. When the reservoir heterogeneity does not favor growing thin acid-etched channels, we investigated how to utilize the acid viscous fingering technique to achieve the channeling etching pattern in such reservoirs. Through numerical simulations, we found that thin acid-etched channels can form inside acid viscous fingers. The regions between viscous fingers are left less etched and act as barriers to separate acid-etched channels. In acid fracturing treatments with viscous fingering, the etching pattern is largely dependent on the perforation spacing. With a proper perforation design, we can still achieve the channeling etching pattern even when the reservoir does not have interbedded dolomite layers.

Download Full-text

Low-viscosity single phase acid system for acid fracturing in deep carbonate reservoirs

MRS Communications ◽

10.1557/s43579-021-00135-8 ◽

2021 ◽

Author(s):

Mohammed Sayed ◽

Fakuen Chang ◽

Amy J. Cairns

Keyword(s):

Single Phase ◽

Carbonate Reservoirs ◽

Acid System ◽

Acid Fracturing ◽

Low Viscosity

Download Full-text

Introduction of a Workflow for Tomographic Analysis of Formation Stimulation Using Novel Nano-based Encapsulated Acid Systems

Istrazivanja i projektovanja za privredu ◽

10.5937/jaes0-29694 ◽

2020 ◽

pp. 1-7

Author(s):

Dmitri Petrakov ◽

Hamed Jafarpour ◽

Jafar Qajar ◽

Hamed Aghaei ◽

Hasan Hajiabadi

Keyword(s):

Chemical Engineering ◽

Carbonate Reservoirs ◽

Petroleum Engineering ◽

Productivity Index ◽

Acid Mixture ◽

Core Analysis ◽

Rock Composition ◽

Novel Technologies ◽

Acid Fracturing ◽

Digital Rock

During the production time, it is crucial to manage the reservoir efficient productivity and keep it at a profitable level. Matrix acidizing in carbonate reservoirs is a common course of action to increase the efficiency of production. The present project is based on an integrated multi-disciplinary plan as an arena to merge traditional and novel technologies in the field of petroleum engineering, petroleum geoscience, chemical engineering, computer vision and mineralogy. Some crucial parameters such as permeability/porosity changes occurred during carbonate acidizing are modelled and analyzed based on various modern technologies, such as, the novel digital rock technologies. A waste variety of nanoparticles is also used in order to design a novel acid mixture for stimulating the carbonate reservoirs. Specifically, this study is considered as a one-step forward in development of smart encapsulated acid systems using a range of hydrophobic silica nanoparticles in various grades of hydrophobicity. Moreover, the present study can be considered as the first practical example for application of digital rock physics in improvement of acidizing operation in Iran and Russia. The proposed research methods are consist of preparation of encapsulated acids, sample and data collection, conventional core analysis, digital core analysis, lab experiments and modelling and conclusion. Characterization of the efficiency of this process was once more characterized using the aforementioned digital rock technologies to visualize the effect of encapsulated acid fracturing operation, impact of surface modification of silica NPs on the etching efficiency, the physical properties of core samples, and subsequently the final productivity index. Thin section, SEM and FE-SEM analysis was then performed to further evidence the efficiency of this method. Moreover, the efficiency of this method was categorized based on the identified mineralogy and rock composition. It was concluded that the dissolution rate was significantly increased as a result of acid neutralization control and the reaction rate decreased which in turn resulted inmore homogenous patterns of wormholes, higher permeability, and so, more successful acid treatment. Thanks to the reduced accessible surface of acid systems caused by their emulsion-based nature, it was found that this novel encapsulation process can reduce the risks of corrosion in all the equipment in surface and bottom hole. It naturally reduces the extra costs of corrosion-related damages and subsequent workover operations, which are the common need of most of the wells treated by conventional acid fracturing operations.

Download Full-text

Analysis of Flow Behavior for Acid Fracturing Wells in Fractured-Vuggy Carbonate Reservoirs

Mathematical Problems in Engineering ◽

10.1155/2018/6431910 ◽

2018 ◽

Vol 2018 ◽

pp. 1-20 ◽

Cited By ~ 1

Author(s):

Mingxian Wang ◽

Zifei Fan ◽

Xuyang Dong ◽

Heng Song ◽

Wenqi Zhao ◽

...

Keyword(s):

Composite System ◽

Numerical Solutions ◽

Transient Flow ◽

Flow Behavior ◽

Outer Region ◽

Carbonate Reservoirs ◽

Single Fracture ◽

Specific Flow ◽

Type Curves ◽

Acid Fracturing

This study develops a mathematical model for transient flow analysis of acid fracturing wells in fractured-vuggy carbonate reservoirs. This model considers a composite system with the inner region containing finite number of artificial fractures and wormholes and the outer region showing a triple-porosity medium. Both analytical and numerical solutions are derived in this work, and the comparison between two solutions verifies the model accurately. Flow behavior is analyzed thoroughly by examining the standard log-log type curves. Flow in this composite system can be divided into six or eight main flow regimes comprehensively. Three or two characteristic V-shaped segments can be observed on pressure derivative curves. Each V-shaped segment corresponds to a specific flow regime. One or two of the V-shaped segments may be absent in particular cases. Effects of interregional diffusivity ratio and interregional conductivity ratio on transient responses are strong in the early-flow period. The shape and position of type curves are also influenced by interporosity coefficients, storativity ratios, and reservoir radius significantly. Finally, we show the differences between our model and the similar model with single fracture or without acid fracturing and further investigate the pseudo-skin factor caused by acid fracturing.

Download Full-text