fundamental studies on natural gas hydrate reservoir development

Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011.

FUNDAMENTAL STUDIES ON NATURAL GAS HYDRATE RESERVOIR DEVELOPMENT PROGRAM BY DEPRESSURIZING Yueming Chen∗, Yaping Liu, Shuxia Li, Zhanxiang Lei College of Petroleum Engineering China University of Petroleum Dongying Shangdong province, 257061 People’s Republic of China

ABSTRACT As an alternative energy resource with huge potential, commercial exploitation of natural gas hydrates (NGH) has received massive attention in recent years. Fundamental studies on NGH reservoir development is key for the commercial exploration of NGH reservoir and are the very advancing steps in NGH development. This paper systematically discusses several challenging issues in this aspect, such as NGH reservoir quality evaluation, production performance analysis and commercial economic evaluation assessment for different quality NGH reservoirs. These studies on NGH reservoir development, which provided the foundation for the development of NGH reservoir, are of great theoretical and practical values. Keywords: natural gas hydrate, fuzzy theory, reservoir quality evaluation model, physical simulation, mathematical model, economic evaluation

μ g gas viscosity [mPa·s]

NOMENCLATURE The nomenclature is listed in alphabetical order. C r specific heat of host rock [J/kg-K]

1. INTRODUCTION

k d dissociation constant [kg/m2·Pa·s] K rg relative permeability of gas [fraction]

With the ever rising demands for oil and gas resources, seeking clean and efficient new energy sources becomes research priority in many countries. At present, Natural Gas Hydrate (NGH) is regarded as a premier high-quality and clean energy source for the future

peq NGH phase equilibrium pressure [MPa] pi initial pressure [MPa] shi NGH reservoir initial saturation [fraction] swi initial water saturation [fraction] Ti NGH reservoir initial temperature [°C]

Many countries have listed NGH reservoir development as their national priority. So far, significant amount of research work has been carried out on characterization of NGH reservoirs, including laboratory based on experiments and

ρ h NGH density [kg/m3] φ 0 reservoir porosity [fraction] ΔH reservoir enthalpy [×106J/kg·k]

∗ Corresponding author: Phone: +86 10-0546 8393014 E-mail: [email protected]

1

numerical simulations. However, research on NGH reservoir development program is relatively weak.

These 12 parameters form the NGH reservoir quality evaluation index.

In this paper, we present a reservoir evaluation model based on reservoir depressurization development program. After developing an assessment model on reservoir quality, a quasi-3D physical model is built to analyze the depressurization development laws obtained from laboratory tests for NGH reservoirs with various qualities. Based on numerical simulations, economic evaluations of NGH reservoir development program can be carried out, which will provide both theoretical foundation and technical support to the future commercial exploitation of NGH reservoirs.

2.1.2 Reservoir quality evaluation system The factors have obviously hierarchical characters. In order to include all the information the multitudinous factors provide, and obtain conclusion conforming to the actual result, the 12 factors are divided into groups, according to their hierarchical characters and fuzzy theory. Setting U to be the set of all selected indexes, i,e a senior evaluation system. Based on factors’ relationships and appraisal results, U is further divided into two matrixes: U = {U1, U2}.

This research has important social significance and academic values, as well as strategic significance. It relates directly to the energy supply, energy safety, and economic and social development.

U 1 = { Ti , pi , φ0 , Cr },

2. NGH RESERVOIR QUALITY EVALUATION

In mathematical term, the senior evaluation system U comprises of two junior evaluation systems geological matrix U1 and fluid matrix U2.

2.1 NGH reservoir quality evaluation model

2.1.3 Reservoir quality classification

2.1.1 Selection of assessment index NGH reservoir itself can be treated as an open, grey, dynamic and complex system, its quality evaluation is inevitable complicate. The more comprehensive the evaluation indexes are, and more diverse its evaluation methods are.

Using the oil field reservoir appraisal method as a reference, NGH reservoir quality are graded by four classes by total scores: 90, which correspond to 4thto 1st quality class of NGH reservoir. Therefore the remark set V = {4th class, 3rd class, 2nd class, 1st class} for fuzzy evaluation is established.

Fuzzy mathematical approach is introduced here to deal with the reservoir assessment. As the relative degree is a fuzzy concept, NGH reservoir quality evaluation is similar to the actual natural process. Below we introduce the fuzzy mathematics method in evaluating the quality of NGH reservoir.

In NGH reservoir quality evaluation process, the factors marking criterions division is complex, furthermore most factors are difficult to be quantified. Taking the oil-gas field evaluation experience as a reference, we define the following scoring system.

According to existing simulation results and expertise’s judgment, 12 factors are selected for NGH reservoir quality evaluation. These are listed as follows following on the factor’s sensitivity from high to low: Ti , shi , peq , pi , ρ h , φ0 , ΔH ,

According to sensitivity analysis of NGH reservoir depressurizing development, 12 dominating parameters are selected as NGH reservoir quality evaluation factors, whose marking criterions are shown as Table 1.

U 2 ={ peq , ρ h , k d , shi , ΔH , s wi , K rg , μ g }

Cr , s wi , k d , K rg and μ g . 70

80

90

95

100

Ti (°C)

16

12~16

8~12

4~8

800

kd (kg/m2·Pa·s)

10−13

ΔH (×106J/kg·k) K rg (m2)

>0.24

0.23~0.24

0.22~0.23

0.21~0.22

0.3

0.25~0.3

0.2~0.25

0.15~0.2