Jianye Mou

Wormhole Propagation Behavior Under Reservoir Condition in Carbonate Acidizing

In carbonate reservoirs, acid is injected into the formation under breakdown pressure to react with the rock to remove the contaminations caused by drilling and production, which is called carbonate acidizing in reservoir development. In carbonate acidizing, acid flows selectively through large pores to create wormholes. Wormhole propagation under experimental condition has been studied by many experts. In this paper, a model which couples a two-scale continuum model simulating wormholing in the invaded zone and a reservoir flow model for the compressed zone was used to study the wormhole propagation behavior under reservoir condition. In this model, the porosity values which are uniformly distributed used in former literature follow the normal law. Based on the model, we first compared the results of the two porosity generation methods, and then studied the wormhole propagation behavior under reservoir condition, and finally simulated a two-layer formation to study the effects of distance and permeability ratio between the two layers. The results show that the normally distributed porosities simulate wormholing better. The effect of compressed zone on wormhole propagation increases with the decrease of compressibility factor and wormhole has a maximal value in length. The effect of distance between the two layers on wormhole lengths and acid distributions can be divided into three zones based on the wormhole length in the lower layer. A critical value of permeability ratio between the two layers exists, below and above which the wormhole length in the low permeability layer decreases sharply and almost keeps constant, respectively.

A New Correlation of Acid Fracture Conductivity Subject to Closure Stress

This paper (SPE 140402) was accepted for presentation at the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, 24–26 January 2011, and revised for publication. Original manuscript received for review 18 November 2010. Revised paper received for review 26 August 2011. Paper peer approved 31 August 2011. Summary The conductivity of an acid-etched fracture depends strongly on void spaces and channels along the fracture resulting from uneven acid etching of the fracture walls. In this study, we modeled deformation of the rough fracture surfaces acidized in heterogeneous formations based on synthetic permeability distributions and developed a new correlation to calculate the acid-etched fracture conductivity. In our previous work, we modeled the dissolution of the fracture surfaces in formations having small-scale heterogeneities in permeability. The characterization of the correlated permeability fields of rock includes the average permeability, normalized correlation lengths in both horizontal and vertical directions, and normalized standard deviation. These statistical parameters have a significant influence on the fracture-etching profiles obtained from the model. Beginning with this fracture-width distribution, we have modeled the deformation of the fracture surfaces as closure stress is applied to the fracture. The elastic properties of the rock, such as Young’s modulus and Poisson’s ratio, have effects on the size of the spaces remaining open after fracture closure. After the model yields the width profile under closure stress, the overall conductivity of the fracture is then obtained by numerically modeling the flow through this heterogeneous system. In this paper, we introduce our models and investigate the effects of permeability and mineralogy distributions and rock elastic properties on the overall conductivity of an acid-etched fracture. A new acid-fracture conductivity correlation is developed on the basis of many numerical experiments.

A New Correlation of Acid-Fracture Conductivity Subject to Closure Stress

This paper (SPE 140402) was accepted for presentation at the SPE Hydraulic Fracturing Technology Conference and Exhibition, The Woodlands, Texas, USA, 24–26 January 2011, and revised for publication. Original manuscript received for review 18 November 2010. Revised paper received for review 26 August 2011. Paper peer approved 31 August 2011. Summary The conductivity of an acid-etched fracture depends strongly on void spaces and channels along the fracture resulting from uneven acid etching of the fracture walls. In this study, we modeled deformation of the rough fracture surfaces acidized in heterogeneous formations based on synthetic permeability distributions and developed a new correlation to calculate the acid-etched fracture conductivity. In our previous work, we modeled the dissolution of the fracture surfaces in formations having small-scale heterogeneities in permeability. The characterization of the correlated permeability fields of rock includes the average permeability, normalized correlation lengths in both horizontal and vertical directions, and normalized standard deviation. These statistical parameters have a significant influence on the fracture-etching profiles obtained from the model. Beginning with this fracture-width distribution, we have modeled the deformation of the fracture surfaces as closure stress is applied to the fracture. The elastic properties of the rock, such as Young’s modulus and Poisson’s ratio, have effects on the size of the spaces remaining open after fracture closure. After the model yields the width profile under closure stress, the overall conductivity of the fracture is then obtained by numerically modeling the flow through this heterogeneous system. In this paper, we introduce our models and investigate the effects of permeability and mineralogy distributions and rock elastic properties on the overall conductivity of an acid-etched fracture. A new acid-fracture conductivity correlation is developed on the basis of many numerical experiments.

Wormhole Propagation Behavior and Its Effect on Acid Leakoff under In Situ Conditions in Acid Fracturing

In acid fracturing, excessive acid leakoff is thought to be the main reason that limits fracture propagation and live acid penetration distance. Although acid leakoff has been studied under experimental conditions, the acid leakoff theory developed under experimental conditions cannot be extended to in situ conditions because the injection rate or pressure drop across a core plug is fixed in the experiments. In this paper, we used a model that couples a two-scale continuum model simulating wormholing in the invaded zone and a reservoir flow model for the compressed zone to simulate acid leakoff process under in situ conditions. Based on this model, we investigated wormhole propagation behavior and its effect on acid leakoff under in situ conditions. The study shows different wormhole propagation behavior under in situ conditions from that under experimental conditions. Wormholes grow fast at the beginning and slow down at later time due to the rise of reservoir pressure caused by the leakoff and the growth of the invaded zone. In oil reservoirs, wormholing has minor effect on acid leakoff because of small compressibility and relatively high reservoir fluid viscosity, but in gas reservoirs, the influence of wormholing on acid leakoff becomes significant due to large compressibility and low reservoir fluid viscosity. Acid viscosity has more notable influence on acid leakoff in gas reservoirs than in oil reservoirs.

Acid Leakoff Mechanism in Acid Fracturing of Naturally Fractured Carbonate Oil Reservoirs

In acid fracturing, excessive acid leakoff is thought to be the main reason that limits fracture propagation and live acid penetration distance. Since most carbonates are naturally fractured, we developed a new model in this paper to simulate acid leakoff into a naturally fractured carbonate oil reservoir during acid fracturing. Our model incorporates the acid-rock reaction, fracture width variation due to rock dissolution on the fractured surfaces, and fluid flow in naturally fractured carbonate oil reservoirs. Given the information of the reservoir, injected acid, and pressure in the hydraulic facture and the reservoir, the model predicts acid leakoff with time. In this study, we found that acid leakoff mechanism in naturally fractured carbonates is much different from that in reservoirs without natural fractures. Widened natural fractures by acid-rock reaction act as high-conductivity conduits allowing leakoff acid to penetrate deeper into the formation, resulting in serious leakoff. Wide natural fractures have a dominant effect on acid leakoff compared to micro-fractures and matrix.

Fractal nature of acid-etched wormholes and the influence of acid type on wormholes

Abstract Two-scale (Darcy scale and pore scale) continuum wormholing models are built to study the fractal nature of the acid-etched wormholes in acidizing carbonate reservoirs and the influence of acid type on the conditions for wormholes to form. This model considers convection-diffusion mass transfer and reaction on the acid-rock interface, and the fractal dimension of dissolution patterns is calculated using box-counting method. The results show that wormholes are formed when convection and diffusion are equivalent in strength; when convection dominates, uniform dissolution is formed; when diffusion dominates, surface dissolution is generated. For the zones where the porosity is greater than 0.7, the fractal dimensions of the surface dissolution, wormholes and uniform dissolution are 1.46, 1.50 and 1.44, respectively, among which, the fractal dimension of the wormholes is approximate to the experimental results obtained by Daccord and Lenormand (1.60.1). The reaction between weak acid and rocks is controlled by reaction kinetics, the acid-etched wormholes are wide, and more acid is consumed. The reaction between strong acid and rocks is controlled by mass transfer, the acid-etched wormholes are narrow, and less acid is consumed. According to an example calculation, it is found that acidizing treatments should be performed in an isolated short section each time when the horizontal well is long.

The Mechanism of Leakoff Reduction of Clean Self-Diversion Acid in Acid Fracturing

Leakoff has been regarded as the main factor that limits fracture propagation and live acid penetration distance in acid fracturing. Reducing acid leakoff is a common measure to increase effective fracture length. Due to cleanness and leakoff reduction, clean self-diversion acid (VES: viscoelastic surfactant acid) has been used in acid fracturing. So far the mechanism of leakoff reduction by VES acid is interpreted qualitatively, and performance evaluation of leakoff reduction is based on laboratory results. However, laboratory results cannot be generalized to formation conditions directly. In this paper, we developed a model to simulate VES acid leakoff. The model consists of two parts: the acid invaded zone and the compressed zone. Coupling of the two zones forms the VES acid leakoff model. In the invaded zone, we built a VES wormhole model, which simulates acid flow, acid-rock reaction, porosity variation, and acid viscosity variation. The VES wormhole model predicts the pressure field, the flow field, acid concentration distribution, cation concentration distribution, VES concentration distribution, and viscosity distribution. For the viscosity variation due to acid-rock reaction, we use a correlation of viscosity as a function of pH, temperature, cation concentration, VES concentration, and shear rate based on experiment results. With the model, by extensive numerical simulation, we analyzed wormhole progation behavior under VES acid flooding and put forth the mechanism of VES acid leakoff reduction. The study shows that wormholing and viscosifying have competitive effect on leakoff. Wormholing decreases flow resistance, while the viscosifying and the increase of the width of spent acid zone raise flow resistance. Wormholes have fewer branches under VES acid compared to regular acid. For reservoirs with small compressibility, for example oil reservoirs, VES acid has relatively small effect on leakoff because compressibility has dominant effect on leakoff ; while for reservoirs with large compressibility, for example gas reservoirs, VES acid has a significant contribution to leakoff reduction because viscosity has a dominant effect on leakoff.