Accurate sectional and differential acidizing technique to highly deviated and horizontal wells for low permeable Sinian Dengying formation in Sichuan Basin of China

The Sinian Dengying formation reservoirs in Sichuan Basin of China have characteristics of high temperature, high pressure, deep buried depth, strong heterogeneity, low porosity and permeability, rich pore, natural fractures, vugs and high concentration H2S, CO2, which limit the reconstruction technologies about the gas reservoir. The formation poroperm of Tainei block Dengying unit is worse than that of Taiyuan block’s, and the difference of gas well productions in Tainei block is enormous. The technique of retarded acidizing with segmented by mechanical system was developed and applied in Dengying unit of Taiyuan block, which significantly raised the gas well production. It is difficult for the retarded acidizing technique to applied to gas wells in Tainei block, for the lower formation quality and immature fractures/vugs. The keys to formation reconstruction for highly deviated and horizontal wells are acid homogeneous placement and deep penetrating in order to connecting natural fracture and vugs system. With exploration and practice over several years, a set of segment acid fracturing comprehensive technologies for highly deviated and horizontal wells were formed, including accurate fragmenting based on quantizing evaluation of formation quality, differential acidizing of retarded acid and self-generation acid. The slow acid fracturing technology is used for the formation which the RES is over 1.0. Then, the multistage alternating injection acid fracturing technology is used for the formation which the RES is lower 1.0. The acid fracturing process parameters, such as displacement and acid strength, were optimized by experiments. The precision acidizing technique was applied for gas well of Tainei block Dengying unit, which the testing production of gas well is over 1 million cubic meter per day after aciding stimulated. The precision acidizing technique significantly raised gas well production of Tainei block Dengying unit and is helpful for exploratory development of Tainei block Dengying unit with low formation quality. According to the characteristics of dispersed and superimposed development of high-quality reservoirs in long horizontal section, the acid fracturing process criterion of "one section and one policy" was established. The acid fracturing process criteria for reservoirs with different transformation potential are established, and the pertinence of acid fracturing transformation is improved. Gel acid fracturing process is adopted for high-quality reservoir, the acid fracturing process of authigenic acid preflush is adopted for medium reservoir and the acid fracturing process of multistage alternating injection of authigenic acid preflush + gelled acid is adopted for low-quality reservoirs. The targeted reconstruction of strong heterogeneity long well section is realized.


Introduction
Sichuan Basin is an important part of the construction of 30 billion cubic meters strategic large gas province of Southwest Oil and Gas Field Company, because of the huge exploration and development potential owing to the rich deep carbonate gas reservoirs, and the proved natural gas reserves in Sinian Dengying Formation of Gaoshiti Area of 2170.81 × 10 8 m 3 through exploration [1,2]. Specifically, a large number of dissolved pores, fractures, caves and their random combinations are developed in Sinian carbonate reservoir, which are characterized by complex reservoir space type, deep burial, strong heterogeneity, high content of H 2 S, CO 2 and other acid gases [3]. However, it is urgent to make breakthroughs in single well production due to the large yield variance.
Further exploration and development promotes the gradual transfer of focus from the platform margin to the low-permeability block in the platform with increasingly complex reservoir geological conditions. In addition, the reservoir stimulation faces many challenges, such as the layering and segmentation for highly deviated/horizontal wells with heterogeneous long construction intervals, the unclear and poorly targeted stimulation corresponding to different reservoir characteristics. Therefore, it is necessary to carry out in-depth research on targeted stimulation technique for highly deviated/horizontal wells in strongly inhomogeneous and low-permeability reservoir on the basis of previous achievements [4][5][6].
The fracture-propagation model coupled these factors as fractures propagate to obtain the fracture conductivity distribution along the length was created to update the domain of the acid model continuously [7]. It is demonstrated how the acid fracturing stimulation using different technologies in multidisciplinary team integration helped un-locking the production potential and setting up development strategy for Tuba tight carbonate reservoir, with case histories [8]. It was described the planning for an application of multistage acid fracturing in a dual lateral well. It also highlights associated risks and methods to be used for risk mitigation. Multistage fracturing (MSF) is considered to be very challenging in dual lateral wells due to the challenges [9].
The wellbore structure characteristics of highly deviated/horizontal wells and their different seepage laws from those of vertical wells determine that their reservoir segments are vulnerable to damage by solid phase of drilling fluid and filtrate. Meanwhile, due to relatively long drilling time, drilling fluids and formation fluids will produce emulsion blockage under certain formation conditions, which also has a great influence on single well productivity [10][11][12][13][14]. The artificial fracture morphology created by acid in carbonate rock was revealed through CT scan. It was found that as acid was more conducive to connection of natural fractures, especially low viscosity gelled acid, hydraulic fractures induced by acid could propagate along natural fractures after initiating from natural fractures, and the process would not be restricted by maximum horizontal stress [15,16]. While the initial hydraulic fracture morphology and variation of acid-rock reaction temperature was also the main influences on the acid-etching pattern and conductivity [17]. Moreover, the effect of reservoir stimulation mainly depends on whether the natural fracture-cave system is connected or whether there is enough drainage radius. Therefore, the reservoir stimulation of highly deviated/horizontal wells mainly focuses on the single well and single interval to achieve targeted reformation, that is, the fine layered segmentation and differential acid fracturing of the well interval with strong heterogeneity construction, so as to maximize the efficiency of gas well reservoir stimulation.

Longitudinal distribution
From the margin to the area in the platform, the reservoir thickness of the fourth member of Dengying Formation (Deng-4 member) decreases as a whole. The reservoir thickness of Moxi-8 block in the platform is dominated by thin reservoirs, which are mainly developed in the upper submember of Deng-4 zone. Thus, according to the drilling experience of the vertical well reservoir in this well area, the vertical thickness of the reservoir in the platform is 31.1 ~ 58.8 m.

Reservoir physical property
Reservoir physical properties of Deng-4 zone in Gaoshiti-Moxi block are worse than those of Deng-4 zone in the margin of platform. The average porosity of cylindrical plug reservoir in Deng-4 member in MX-E block is 3.3%; the average permeability is 0.48mD, and the permeability distribution is less than 0.01mD, so the reservoir is characterized by low porosity and permeability (Figs. 1 and 2).

Reservoir space
The fracture-cave system in Moxi-8 block in the platform is not developed as well as that in the platform margin reservoir, which is dominated by small holes and caves (small caves 88.3%, medium caves 9.2%, and large caves 2.5%). The natural fractures are mainly map cracks and highangle cracks, which are mainly used to connect holes and caves and improve the permeability of reservoir.
3 Fine segmentation and differential acid fracturing technique system for highly deviated/horizontal wells

Optimization of working fluid system
(1) Main acid system The comparison of the acid-etched fracture conductivity of 20% high-temperature gelling acid, authigenic acid and 20% high-temperature organic diverting acid shows that injecting acid into the fractures that are opened can achieve effective etching, form acid-etched fractures, and improve the conductivity. As the closing pressure increases, the conductivity of acid-etched fractures decreases rapidly, as shown in Fig. 3. Because of the stronger penetrability and better conductivity of acidetched fractures, gelling acid is selected as working fluid system for acid fracturing.
(2) Optimization of preflush liquid: authigenic acid system The non-uniform etching of reservoir core by combination of gelling preflush + gelling acid and authigenic acid preflush + gelling acid was simulated by experiment to compare and analyze the acid-etched fracture conductivity of them when the closing pressure increases from 5 to 50 MPa (Fig. 4). The results show that under the same closing pressure, the acid etching fracture conductivity of the latter is slightly better than that of the former, and the formation pre-treatment with authigenic acid preflush can reduce the fracture pressure, which is suitable for  According to the reservoir characteristics, such as high construction risk resulting from high Young modulus and Poisson's ratio of the reservoir, well-developed natural micro-fractures, severe work fluid loss (high viscosity), connection is favorable to the reservoir area (long fracture). Liquid performance: The acidizing fluid system can effectively reduce the construction risk, while the use of highviscosity authigenic acid preflush can effectively reduce fluid loss and form long artificial fractures.

Fine segmentation method for heterogeneous reservoirs
Based on reservoir physical properties, logging data and test production of pre-constructed wells, partial least squares (PLS) was used to determine the relationship between test production and multiple reservoir physical parameters, and to determine the correlation degree of reservoir physical parameters, and to measure the correlation degree between variables by introducing Pearson product-moment correlation coefficient. According to reservoir characteristics of highly deviated/horizontal wells, 11 main influencing factors reflecting reservoir physical properties were selected, including effective reservoir thickness, vertical reservoir thickness, reservoir I + II ratio, hole deviation angle, seepage loss, all gas hydrocarbon, depth-shallow resistivity ratio, porosity, permeability, water saturation and reservoir coefficient Hϕ.
Firstly, dimensionless treatment of these 11 parameters is carried out by formula (1): The dimensionless coefficients of each influencing factor were substituted into the following formula to obtain the Pearson correlation coefficient: where r = correlation coefficient; m = number of sample point; X a,i = coefficient of influencing factor a corresponding to the i-th sample point.
Multivariate linear regression was performed using PLS to determine the impact of each factor on the test production and to define the variable importance in projection I vip,j . (1) where I vip,j = variable importance in projection of influencing factors on test production;R d y;t i = interpretability of a single principal component t i to dependent variable y;R d y;t 1 , t 2 , ⋯ , t h = cumulative interpretability of all principal components to y;p = the number of independent variables. Based on the above formulas, the variable importance in projection of each variable to the test production is obtained, as shown in Table 1 as follows: To sum up, a formula for calculating the reservoir stimulation factor is obtained to quantitatively evaluate the reservoir quality of highly deviated/horizontal wells: In view of strong reservoir heterogeneity, large construction interval and span, and difficult targeted stimulation of highly deviated/horizontal wells, fine laying and segmentation were realized with the employment of PLS and variable importance in projection. Besides, sample points such as highly deviated/horizontal wells were updated to determine the weighting coefficient of each factor, so as to obtain the calculation method of reservoir stimulation coefficient: This method can be used to quantitatively evaluate the quality of reservoir stimulation and its heterogeneity, providing basis for fine laying and segmentation of highly deviated/horizontal wells and targeted reformation process, because it overcomes the difficulties such as large  (1) The technique of acid fracturing with retarded acid

Differential acid fracturing technique of highly deviated/horizontal wells
Gelling acid reduces fluid loss to some extent due to its high acid viscosity, which reduces acid-rock reaction, improves the retention time of active acid, and increases the ability of deep formation reformation. At the same time, the alternating injection of preflush and gelling acid has the effect of pre-cooling formation and blocking acid etching holes by preflush, which further reduces the acidizing fluid loss and the speed of acid-rock reaction. Gelling acid fracturing technique has played a certain role in increasing the action distance of acidizing fluid, breaking through the pollution zone, fracturing the reservoir, dredging the natural fractures and holes, and improving the construction effect.
(2) The technique of acid fracturing with preflush The technique of acid fracturing with preflush is to make fractures with preflush and form filter membrane on the fracture surface at the same time, so as to reduce fluid loss and formation temperature, reduce acid-rock reaction speed and expand the effective action distance of acidizing fluid. The injection of the second stage preflush fills the acid etching caves and increases the acid etching distance. Depending on the viscosity difference between the preflush and the acidizing fluid, the acidizing fluid will finger in the fracture to produce etching grooves, improve the fracture conductivity, and then improve the productivity. The mechanism of multistage alternating injection acid fracturing is similar to that of preflush acid fracturing, differentiated by the facts that preflush and acidizing fluid are injected alternately for many times, resulting in multiple temperature reduction to formation and formation of filter cakes, which significantly reduces the rate of fluid loss in the latter injection compared with the previous one. At the same time, acid has formed viscous finger advances in the preflush for many times, resulting in features with a larger scale and higher conductivity.
Experimental simulation shows that the conductivity of etched fractures is different under different injection displacement. By comparing the conductivity of acidetched fractures obtained by 100 mL/min and 150 mL/ min injection displacement, the conductivity increases with the increase of injection displacement, indicating that the larger the construction displacement, the stronger the conductivity, and the better the reservoir stimulation (Fig. 5).
(3) Process optimization mode of targeted reformation According to different reservoir characteristics, fracturecave development and different reservoir quality, differential acid fracturing technique of single well and single interval with "one strategy for one interval" has been formed to match acid fracturing techniques with geological target, strengthen pertinence of single well and single interval reservoir stimulation and improve the reformation efficiency. By quantitatively evaluating the reservoir quality of highly deviated/horizontal wells, the reservoir stimulation requirements and targeted reformation processes of different grades are further refined (Tables 2, 3).
In view of the strong heterogeneity of the reservoir in high-inclined wells/horizontal wells and the long construction interval, the single well and single interval are targeted for reformation combined with the evaluation of the reservoir stimulation coefficient. The different acid fracturing techniques are used in the corresponding intervals at different reservoir stimulation coefficients, and the single well and single interval acid fracturing design parameters are optimized to achieve accurate reformation of the single well and single interval (Table 4).

Field test
Differential acid fracturing technique of highly deviated/ horizontal wells with one strategy for one interval has been developed. By means of evaluating reservoir stimulation coefficient, the acid fracturing with authigenic acid preflush and gelling acid fracturing processes have been matched, respectively. The average test production of vertical wells in Moxi-8 block in the platform is 59,000 square meters per day, that of highly deviated wells is 473,500 square meters per day, and the test production of single well is increased by 7.6 times, which achieves the increase of single well productivity (Fig. 6, Tables 5 and 6).

Conclusions
(1) The key to get a high production of gas wells in Sinian low-permeability reservoir is to effectively connect the natural fracture-cave system of immediate vicinity of wellbore. Through using fracture-cave monitoring technology, in combination with the direction of artificial fracture, the probability of connecting fractures or caves could be increased, and then the production of gas well could be effectively increased. (2) In view of the difficulties faced by low-permeability reservoirs in Moxi structure in the platform, such as low porosity, low permeability, low development degree of fracture and cave, and the difficulty of effective utilization of reservoirs by conventional acid fracturing techniques, it is made clear to improve the coverage of artificial fractures and there formation objectives of effectively improving gas well production. (3) Based on the evaluation of reservoir stimulation coefficient, a fine segmented differential acid fracturing technique is formed for highly deviated wells, and the acid fracturing design parameters are defined. After three field tests, it is found that the test production is 7.1 times higher than that of the vertical wells in the same block, and remarkable improvements have been achieved. (4) According to the characteristics of high pump pressure, low displacement, high construction pressure, high shut-down pressure and high acid absorption pressure gradient in the low-permeability reservoir reformation of Deng-4 member, the tests with deep acid fracturing technique and technique of acid fracturing with preflush were carried out in order to make long fractures, and the transformation effect was achieved to a certain extent. The main technology of