Journal of Power and Energy Engineering
Vol.07 No.07(2019), Article ID:94043,11 pages
10.4236/jpee.2019.77004

Study on Production Regulation of Reservoir under Different Demand

Jie Tan*, Enhui Sun, Wei Wang, Jingmin Guo, Wei Yang

Bohai Oilfield Research Institute of CNOOC Ltd.-Tianjin Branch, Tianjin, China

Copyright © 2019 by author(s) and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).

http://creativecommons.org/licenses/by/4.0/

Received: April 15, 2019; Accepted: July 28, 2019; Published: July 31, 2019

ABSTRACT

Natural gas production is related to the demand for gas, which is low in summer and high in winter. While the gas storage is still being demonstrated and constructed, oil and gas fields should formulate and implement production control schemes suitable for gas reservoirs. The realization of natural gas production can not only meet the demand of gas consumption, but also ensure the scientific and efficient development of gas reservoirs, and meet the needs of dynamic analysis of gas reservoirs at different development stages and scientific research of gas reservoirs. In this paper, KAPPA Workstation 5.20 software is used to determine the inflow dynamic model of a single well. The nodal method is used to determine the reasonable production and peak shaving capacity in combination with the critical fluid carrying capacity of gas wells and the erosion rate of gas wells. The reasonable production allocation in each period, i.e. the production control scheme, is obtained. It solves the scientific and efficient development of natural gas in X gas field, which is still under the construction of gas storage, and provides guidance for gas reservoir development management and regulation.

Keywords:

Gas Reservoir, Regulation of Production, Erosion Velocity, The Critical Liquid Carrying Capacity of Gas Well, Reasonable Production

1. Introduction

The surplus production capacity of a gas field in winter is insufficient, and the demand of downstream gas field in summer is relatively small. The underground gas storage is only in the demonstration construction, which needs to be allocated monthly through production control. In order to coordinate the contradiction between gas field production and market demand for gas, most of the solutions at home and abroad are obtained by building some surplus capacity, underground gas storage and introducing LNG. A lot of work has been done at home and abroad on the production control of different types of gas reservoirs using gas storage. There are abundant reference materials available.

H. Glen Van Horn put forward the optimum design method for four systems of cushion gas, well, purification equipment and compressor power according to the optimum investment demand of gas storage. Robert A. Wattenbarger established the design method for maximizing natural gas extraction from gas storage during peak demand season. R. P. Anderson applied tracer to study gas migration characteristics of gas storage. I. Lakatos described the pressure in gas storage. The method of controlling water infiltration and improving rock stability around production wells and injection wells at the same time is proposed [1] [2] [3] [4] . Research on domestic gas storage technology has also been accelerated in recent years. Fang Liang put forward the application of nodal analysis method to design the gas injection system of gas storage; Tan Yufei put forward the dynamic operation analysis method of injection and production wells in water drive gas storage, established the prediction model of gas storage engineering for peak shaving and optimal operation of gas storage, and put forward the method of calculating the leakage of gas storage; Chen Jiaxin applied grey according to the grey characteristics and incompatibility of factors affecting gas storage. Matter element analysis method is used to determine the reserves of gas storage, and optimization design method for gas injection process of gas storage is put forward [5] - [12] .

However, there are few studies on production regulation and control of gas storage when the scale of gas storage is insufficient or the scale of gas storage is insufficient, which is different from that of general gas storage. The main manifestation is that production regulation and control of gas storage when the scale of gas storage is insufficient or the scale of gas storage is insufficient requires one-off production capacity, peak shaving capacity and gas reservoir production of all gas wells. According to different monitoring objectives, research work should be carried out in every well. Peak shaving ability of production regulation decreases gradually with the increase of production time. Its production regulation scheme must be optimized on the premise of meeting the demand for natural gas production.

The gas storage of X gas reservoir is still in the process of demonstration and construction. Based on the gas reservoir percolation theory, well test analysis theory and material balance principle, this paper establishes the theory and method of Non-shut-in productivity analysis to determine gas well productivity by using production dynamic data. By analyzing the production constraints of gas wells in gas reservoir, the production control scheme of X gas reservoir is obtained, which provides guidance and technical support for gas reservoir development management and control.

2. Analysis of Gas Reservoir Productivity

Analyzing the productivity of a single well, this paper mainly studies two parts: one is to analyze the production constraints of a single well; the other is to analyze the productivity of a single well.

2.1. Production Constraints of Gas Wells

The production constraints of gas wells mainly include two aspects: one is that the high-speed flowing gas erodes the tubing and string to become the maximum velocity of erosion; the other is that the minimum velocity of fluid carrying in the wellbore to the surface is called the minimum velocity of fluid carrying.

2.1.1. Erosion Velocity of Gas Wells

Erosion velocity is the maximum limit flow rate to avoid the erosion effect of high-speed flowing gas in wellbore on tubing and tubing string. The safe flow rate of erosion can be calculated by formula (1) Beggs [13] :

{ V e = C ρ g 0.5 q e = 40538.17 × D 2 ( p w h Z T γ g ) 0.5 (1)

where, V e is erosion velocity, m/s. C = 122 . ρ g is gas density, kg/m3; q e is Erosion discharge, m3/d; A is Tubing section area, m2. pwh is wellhead pressure, MPa; T is wellhead temperature, K; Z is deviation coefficient; γ g is relative density of natural gas.

2.1.2. Critical Liquid-Carrying Capacity

The minimum flow velocity that brings the fluid accumulated in the wellbore to the surface is called the minimum liquid carrying velocity or the critical production rate. Keeping gas well production greater than the minimum liquid carrying rate is of great significance for improving gas reservoir recovery, especially for water-producing gas reservoirs.

Based on Duggan’s idea of critical flow rate, Turner deduced the critical flow rate formula (2) and the critical flow rate formula (3) of gas wells in 1969 by assuming that the droplet shape carried by high-speed airflow is spherical and analyzing the force acting on the spherical droplet. Turner model is suitable for wellhead pressure greater than 3.4475 MPa [14] .

Critical liquid-carrying velocity:

u c r = 2.5 [ σ ( ρ 1 ρ g ) ρ g 2 ] 0.25 (2)

q c r = 2.5 × 10 4 A p w h u c r Z T (3)

where, u c r is Critical velocity of liquid carrying in gas wells, m/s. σ is Air-water interfacial tension, N/m. ρ 1 is Density of liquids, kg/m3. ρ g is Density of gases,

kg/m3, ρ g = 3.4844 × 10 3 γ g p Z T ; q c r is Critical fluid carrying capacity of gas wells,

104 m3/d; A is Tubing area, m2; p w h is Bottom hole pressure, MPa. T is temperature, K; Z is gas deviation coefficient. Because water is a critical liquid-carrying velocity controlling fluid, the surface tension of gas and water can be calculated, σ = 60 × 10 3 N / m .

2.2. Evaluation of Gas Well Productivity

The formation inflow dynamic model reflects the formation supply capacity of gas wells. The inflow dynamic model describing gas flow from reservoir to bottom hole can be described by exponential and binomial equations. In this study, binomial productivity equation is used to study gas flow dynamics [15] .

P R 2 P w f 2 = A q s c + b q s c 2 (4)

where P R is Formation pressure. P w f is Bottom hole flow pressure. A is Laminar Flow Coefficient. B is Turbulent Flow Coefficient. q s c is Gas well production.

Formula (4): The first item on the right represents the pressure loss caused by viscous flow and the second item represents the pressure loss caused by inertia. The sum of the two losses constitutes the total pressure drop of the gas flowing into the well. When the infiltration rate is linear, the first term (Aqsc) plays a major role, while the second term (Bqsc) can be neglected. There is a linear relationship between pressure square difference and production; when the infiltration rate increases or the infiltration rate is multiphase flow, the influence of the second inertial resistance must be considered, and the pressure square difference has a non-linear relationship with production.

In this study, using KAPPA Workstation 5.20 software, TOPAZE module production performance data fitting results (Figure 1), using Saphir module to

Figure 1. Fitting chart of gas well production history.

Figure 2. IPR curve of gas well.

obtain the IPR curves of gas wells at different production times (Figure 2), in order to study the current inflow performance of individual wells in each gas reservoir.

Gas well outflow dynamic curve is an outflow dynamic curve, also known as tubing dynamic curve [15] , which reflects the productivity of gas wells when wellhead pressure is a constant. The outflow performance curves of gas wells can be obtained by choosing the bottom hole flowing pressure calculation formula (formula 5) [15] according to the nature of different types of gas wells (tubing size). Obviously, the outflow performance curve of gas wells is related not only to wellhead pressure, but also to tubing size and gas well production.

p w f = p t f 2 e 2 S + 1.324 × 10 18 f ( Q s c T ¯ Z ¯ ) 2 D 5 ( e 2 S 1 ) (5)

Gas well outflow performance curve is an outflow performance curve, also known as tubing performance curve, which reflects the productivity of gas wells when wellhead pressure is a constant (Figure 2).

When the parameters change, the stable working conditions of gas wells will change. Therefore, the maximum production capacity of a single well can be evaluated by calculating the inflow and outflow dynamic curves of each gas reservoir, which reflects the limit working state of a gas well.

The maximum production capacity of gas wells can be solved by production system analysis (also known as nodal analysis). Generally, bottom hole is chosen as the solution node, and the well production system is divided into two parts: the seepage section from reservoir to bottom hole (inflow part, controlled by reservoir seepage characteristics and formation pressure) and the tubular flow section from bottom hole to wellhead (outflow part, controlled by wellbore characteristics

Figure 3. Reasonable determination of production map for gas wells.

and wellhead oil pressure). The bottom hole flow pressure is the coordination point between inflow and outflow parts (Figure 3). The purpose of dynamic analysis of inflow and outflow is to maximize the coordination between formation supply capacity and wellbore production capacity.

3. Study on Rational Production Allocation of Gas Reservoirs

Reasonable production of gas wells is one of the important indicators of gas reservoir development, which is of great significance to the efficient development of gas reservoirs. According to the specific conditions of gas reservoirs, sometimes the reasonable production of gas wells can be determined by single factor, and sometimes a variety of factors should be considered to determine the reasonable production synthetically, so as to ensure the smooth and safe gas supply of gas wells [16] .

Based on the previous research and the actual situation of gas reservoir, the principle of determining reasonable production is put forward (Figure 3):

1) The maximum production capacity is greater than the critical liquid carrying capacity and less than the erosion rate. The allocated production capacity is just between the maximum production capacity and the critical liquid carrying capacity. The output calculated by node method is regarded as reasonable output.

2) The maximum production capacity is greater than the critical liquid carrying capacity and the erosion velocity. The output calculated by nodal method is larger than the erosion rate, and the erosion rate is regarded as the reasonable output.

3) The maximum production capacity is greater than the critical liquid carrying capacity, and the allocated production capacity is less than the critical liquid carrying capacity which is served as a reasonable output.

4) The maximum production capacity is less than the critical liquid carrying capacity, and the allocated production capacity is less than the maximum production capacity. Maximum production capacity as a reasonable production, but measures must be taken to reduce wellbore fluid accumulation.

5) The maximum production capacity is less than the critical liquid carrying capacity, and the allocated production capacity is greater than the maximum production capacity. Measures must be taken to develop it.

Among them, production allocation is the production obtained by taking reservoir conditions into account and determining factors such as rice production index, pressure difference, thickness and skin.

Routine peak shaving capacity = critical liquid carrying capacity − reasonable output.

Emergency peak shaving capacity = maximum production capacity − reasonable output.

Target yield at low peak stage = reasonable yield-coefficient 1 × conventional peak-shaving ability.

Target yield at peak period = reasonable yield + coefficient 2 × emergency peak-shaving ability.

The allocation coefficient 1 and 2 are put forward here. The allocation coefficient is the ratio of output to peak-shaving capacity that needs to be adjusted. The allocation coefficient 1 and 2 are the allocation coefficients of low peak period and high peak period respectively. The range of values is 0 - 1. If the coefficients exceed the range of values, the target output will not be achieved.

4. Example

Taking X gas reservoir as an example, this gas reservoir has an average porosity of 15%, a permeability of 15 mD, and a total of 9 production wells (Table 1). The daily gas production is 100 × 104 m3/d, and the monthly gas demand varies. The lowest is 70 × 104 m3/d in summer and 180 × 104 m3/d in winter. The construction of gas storage is still under discussion. This requires regulation of production.

Table 1. Specific parameters of X gas reservoir in wells.

4.1. Comprehensive Analysis and Research on Rational Production Allocation

According to the nodal analysis principle, the maximum production capacity and erosion velocity of each gas reservoir at 7.2 MPa of wellhead outflow pressure (gas reservoir wellhead pressure) are calculated. At the same time, the reasonable production allocation is obtained by combining the production allocation (Table 2). Among them, well 2, 3 and 4 are all allocated production less than critical fluid carrying capacity. The critical fluid carrying capacity is selected as reasonable production. The maximum production capacity of well 5 is larger than critical fluid carrying capacity, and the maximum production capacity is selected as reasonable production. Meanwhile, measures are taken to control fluid carrying capacity.

4.2. Research on Peak-Shaving Ability at Present

The peak shaving capacity of each well is obtained according to the reasonable production allocation research in Section 2.

The monthly planned gas production of X gas reservoir is shown in Figure 4 below.

Based on the reasonable production of each gas reservoir determined in Table 3 and the demand for different months, the reasonable production of each well in different months can be determined

4.3. Scheme Design under Reasonable Scheduling Conditions

Reasonable scheduling of gas reservoir production is to satisfy the reasonable scheduling of gas field while controlling production. According to the peak shaving capacity of each gas reservoir and the need of production regulation and control at present, and taking into account the reasonable production of each gas well, the production allocation scheme of gas well under the condition of reasonable dispatch is formulated.

Table 2. Results of single well production of X gas reservoir (yield and productivity unit: 104 m3/d).

Taking the peak in January and the minimum peak in July as examples, on the basis of reasonable production, the production is allocated according to the conventional peak shaving capacity. July is the low peak period, the monthly demand and output is 24.8 million square meters, that is, the daily demand and output is 80 × 104 m3/d, and the current reasonable output can reach 95.44 × 104 m3/d. Because of the lack of gas storage, it is necessary to reduce production according to the conventional peak shaving capacity. The production reduction factor of wells A1, A6, A7, A8, A9 and A10 can be calculated as 0.326 according to the previous definition. Similarly, January is the peak period, but the current reasonable output cannot meet the demand. It is necessary to add 0.544 times emergency peak shaving capacity on the basis of reasonable output. Reasonable allocated yield of the highest and lowest peaks (Table 4).

Figure 4. Monthly gas production of X gas reservoir.

Table 3. Peak regulation capacity of X gas reservoir (output and capacity unit: 104 m3/d).

Table 4. The production of gas well under reasonable scheduling.

5. Conclusions

Comprehensive reservoir engineering, seepage mechanics and other aspects, application KAPPA Workstation 5.20 software, the production control of X gas reservoir is studied, and the following conclusions and understandings are obtained.

1) The limits of peak shaving for gas reservoirs are given by means of minimum fluid carrying capacity, erosion velocity and maximum production capacity.

2) The conventional peak-shaving capacity and emergency peak-shaving capacity are defined by allocating output, erosion speed, minimum liquid-carrying capacity and maximum production capacity. The formula for calculating the target output in a special period is given.

3) Taking X gas reservoir as an example, the reasonable production allocation of each gas well during peak period and low peak period of demand is calculated.

4) It provides a method of monthly production allocation through production control for underground gas storage only in the special period of demonstration construction.

Through the study of gas reservoir production regulation and control, when the gas reservoir is still demonstrating the construction or the scale of the gas reservoir is insufficient to build the gas reservoir regulation, this paper provides a plan for the oil and gas field to formulate and implement the production regulation and control scheme. The realization of natural gas production can not only meet the gas demand, but also ensure the scientific and efficient development of the gas reservoir, and meet the needs of gas reservoir dynamic analysis and gas reservoir scientific research in different development stages.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

Cite this paper

Tan, J., Sun, E.H., Wang, W., Guo, J.M. and Yang, W. (2019) Study on Production Regulation of Reservoir under Different Demand. Journal of Power and Energy Engineering, 7, 39-49. https://doi.org/10.4236/jpee.2019.77004

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