Drill stem and Deliverability test analysis: Case study of a gas field well test analysis in Myanmar By AUNG PHYO MIN Final report submitted in partial fulfilment of the requirements for the Bachelor of Engineering (Hons) (Petroleum Engineering) December 2013 Universiti Teknologi PETRONAS Bandar Seri Iskandar 31750 Tronoh Perak Darul Ridzuan ABSTRACT Drill-stem tests are performed to confirm or prove the presence and the producibility of oil and gas that is detected by the other services. It is usually performed on exploration wells are often the key to determining whether a well has found a commercial hydrocarbon reservoir. Reservoir parameters specifically related to productive capacity such as pressure, permeability can be determined through drill stem test. Common sequence of a drill stem test includes of a short flow period mostly five or ten minutes, which is followed by a buildup period of about an hour that is used to determine initial reservoir pressure. Afterwards, the well is allowed to flow for next four to twenty four hours to establish stable flow to the surface then the well is shut in again for final shut in or build up test which is used to determine permeability thickness and flow potential. Drill stem tests are usually combined with deliverability tests which is referred to the testing of a well to measure its production capabilities and flow performance relationships. Most common deliverability tests are flow after flow, single-point, isochronal and modified isochronal tests. Two main applications of deliverability tests are obtaining the absolute open flow (AOF) potential and generating reservoir inflow performance relationship (IPR) or gas backpressure curve. This paper discusses the case study of drill stem test and deliverability test done on a well in one of the gas fields located in Myanmar. Topics will cover the analyses of reservoir parameters through initial build up test, final build up test during the drill stem test and analysis of flow after flow test for Darcy and Non Darcy skin factors. In addition, it will further discusses the comparison of deliverability tests (empirical and analytical) and lastly perform production forecasting. The analyses are performed through Pansystem which is the well test analysis software developed by Weatherford Inc. KEYWORDS: Drill stem test, Deliverability test, Analysis, Reservoir flow capabilities, Myanmar, Gas field, Pansystem 1 TABLE OF CONTENTS Content Page ABSTRACT 1 Chapter – 1: INTRODUCTION 1.1 Background of Study 5 1.2 Problem Statement 6 1.3 Objectives 6 1.4 Scope of study 6 CHAPTER – 2: LITERATURE REVIEW 7 CHAPTER – 3: METHODOLOGY 3.1 Methodology Diagram 9 3.2 Analysis Process Diagram 10 3.3 Project Activities 11 3.4 Tools Required 11 3.5 Gantt Chart 11 3.6 Milestones 12 CHAPTER – 4: THEORETICAL BACKGROUND 4.1 HORNER’S APPROXIMATION 13 4.1.1 Reservoir Pressure 13 4.1.2 Permeability 14 4.1.3 Formation Damage 14 4.2 Type Curves 15 4.3 Theoretical Deliverability Equations 19 4.4 Empirical Deliverability Equations 21 4.5 Stabilization time 21 2 TABLE OF CONTENTS Content Page CHAPTER – 5 : INPUT DATA 5.1 Rreservoir Description 22 5.2 Layer Parameters Data 22 5.3 Well Parameters Data 22 5.4 Fluid Parameters Data 23 5.5 Gas Composition Data 23 5.6 Layer Boundaries Data 24 5.7 Model Parameters 24 5.8 Rate Changes Data 24 CHAPTER - 6: RESULTS AND DISCUSSIONS 6.1 Test Overview Plot 25 6.2 Initial Build-Up Analysis Using Semi Log Plot 26 6.3 Last Build-Up Analysis Using Semi Log Plot 27 6.4 Type Curve Analysis on Last Build-Up Test 28 6.5 Theoretical Deliverability Analysis 30 6.6 Empirical Deliverability Analysis 31 6.7 IPR Curve 32 6.8 Production Forecasting 33 CHAPTER -7: CONCLUSION AND RECOMMENDATION 34 ABBREVIATIONS AND NORMENCLUTURES 35 REFERENCES 36 3 LIST OF FIGURES Figure Page Figure 3.1: Methodology Diagram 9 Figure 3.2: Analysis Process Diagram 10 Figure 3.3: Milestones Diagram 11 Figure 4.1: Pressure Derivative, Pressure Change Vs Time 16 Figure 4.2: Bourdet + Gringarten type curve 17 Figure 4.3: Type curve analysis using Bourdet + Gringarten type curve 18 Figure 6.1: Test Overview Plot 25 Figure 6.2: Initial Build-Up Semi Log Plot 26 Figure 5.3 : Last build-up Semi Log Plot 28 Figure 6.4: Type curve analysis for last build up 28 Figure 6.5: Theoretical deliverability analysis (LIT analysis) 30 Figure 6.6: Empirical deliverability analysis 31 Figure 6.7: IPR Curve 32 Figure 6.8: Production Forecasting Plot 33 4 CHAPTER-1: INTRODUCTION 1.1: BACKGROUND OF STUDY In this case study two build up tests, and flow after flow tests in between are performed during the drill stem test on gas well. Two build up tests are used to determine the initial reservoir pressure and other parameters such as wellbore storage, permeability and kh. Flow after flow tests are used for Darcy and non-Darcy skin factors estimate then followed by performing deliverability analysis through analytical and empirical methods. Pressure build up tests are the most common well transient tests and conducted by producing a well at constant rate for some time, shutting the well in (usually at the surface), and recording the pressure (usually downhole) in the wellbore as a function of time. From which, formation permeability, current drainage area pressure, damage or simulation characterization and reservoir heterogeneities are estimated. There are many graphical methods to analyze build up tests namely, Semi Log plot analysis, Log-Log analysis and Cartesian analysis as well as type curves analysis. Semi Log and Log-Log analysis are used in this case study to analyse the build up tests. Semi log plot analysis is also known as Hornor plot analysis and like most of other analysis, this analysis is based on assumptions that the reservoir is acting as an infinite, homogeneous, isotropic reservoir containing a slightly compressible, single-phase fluid with constant fluid properties. Wellbore damage or stimulation if there is such, is considered to be concentrated in a skin of zero thickness at the wellbore. In fact, no actual build up test can be modeled according to this description, hence, there is deviation between actual test results and this analysis result. One fundamental assumption for this analysis is that if the well has been producing, most recent rate must be maintained long enough than the second last rate. Only then, it is correct to continue plotting build up test data in Hornor pseudoproducing time vs sandface pressure to estimate formation permeability, original reservoir pressure Pi and skin factor s. Type curves analysis is used to validate the results obtained from Hornor approximation. 5 1.2: PROBLEM STATEMENT All the reservoir parameters calculations are based on predictions using geophysical and geology data or certain methods. Human Error or technical error can be present in these predictions hence, consequently drawing the risk of getting inaccurate reservoir parameters. Therefore, getting the results from one particular analysis or method is not sufficient and should not be relied on unless they are validated against other analysis or method. Therefore, in this project, acquired data obtained from one analysis is validated with other methods. This project is also pursued to enhance the understanding on the area of drill stem test and the well test analysis as a whole in regard of personal interest. 1.3: OBJECTIVES The purposes of the study are as follows, 1. To enhance understanding on drill stem test in gas wells. 2. To enhance understanding on theoretical background of build up tests, type curves, theoretical and empirical deliverability analysis. 3. To acquire and validate the important reservoir parameters. 1.4: SCOPE OF STUDY Works of many authors on build up tests and flow after flow tests will be mainly studied. Basic theoretical background of all the analyses used will also be studied extensively. Type curves will be studied selectively. More importantly, gas well testing and significant terms and derivations are also in the scope of study. will This report will be carried out with the aid of well test software, Pansystem which is developed by Weatherford and thus, this software need to be studied during the mean time. 6 CHAPTER-2: LITERATURE REVIEW Drill stem tests are widely used to determine the producible fluid content of a formation and to determine the ability of a formation to produce. Drill stem test or temporary completion can be performed in both open hole and inside casing through perforations. Drill stem tests are usually performed in potential productive interval which is predicted by logging and core data. Under this method, a test will usually made after penetrating a few feet into the prospective zone and if the results are favourable, subsequent tests may be made in search for fluid contacts (Black, W.Marshall, 1965). Three main components of a drill stem test tool are the test valve, the by-pass valve and the packer. Drill stem tests are usually made up with initial build up period followed by multiple constant flow period and lastly the final build up. The analysis of these flows (draw-down) and shut-ins (build-up) permit the calculation of reservoir parameters such as initial reservoir pressure, permeability, skin, damage ration, radius of investigation, and estimation of absolute open flow potentials in gas wells. In fact, drill stem tests also consist of the preflow period which is the initial flow period after the test depth has been reached and the packers set. The objective of this period is mainly to release the hydrostatic pressure trapped when the packers are set and to discharge the mud contained in the rat hole between the formation and the test valve.