The SPE Foundation through member donations and a contribution from Offshore Europe

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1 Primary funding is provided by The SPE Foundation through member donations and a contribution from Offshore Europe The Society is grateful to those companies that allow their professionals to serve as lecturers Additional support provided by AIME 1 Society of Petroleum Engineers Distinguished Lecturer Program 1

2 Effects Of Complex Reservoir Geometries And Completion Practices On Production Analysis In Tight Gas Reservoirs Stuart A. Cox Marathon Oil Company Society of Petroleum Engineers Distinguished Lecturer Program

3 Outline Introduce production analysis Reservoir geometries - Synthetic cases - Field examples Completion parameters - Synthetic cases - Field examples 3

4 Purpose Address the following question: When performing production analysis, can complex reservoir geometries and completion practices cause linear flow, limited fracture half lengths and limited drainage areas to be predicted? 4

5 Conditions Reservoir geometries Stress dependent permeability Radial composite Two-layer system Completion parameters Hydraulic fracture clean-up and damage Liquid loading 5

6 Production Analysis Rate, time, pressure analysis Long term pressure drawdown test Type-curve matching technique Major Assumptions - Single-phase fluid - Constant reservoir / completion properties - Volumetric production - Bottomhole pressure known 6

7 Applications of Production Analysis Determine effective drainage volume Estimate drainage area Estimate reserves / productive life Identify infill drilling potential Estimate reservoir flow capacity Completion performance evaluation 7

8 Example of Reservoir Flow Geometry on Diagnostic Type Curve 100 Infinite Conductivity Fracture Kh = 3.27 md-ft X f = 200 ft Boundary Dominated Uniform Flow 10 Boundary Dominated Linear Flow Infinite Acting Flow P wd t DA 8

9 Field Example of Flow Characteristics 100 East Texas Example East Texas 2 to 1 Example Rectangular First Boundary 90 Days at 1,017 Days 10 Kh = 3.3 md-ft Inifite Acting Pseudo Radial Flow Boundary Dominated Flow P wd or P wd ' 1 Uniform Flux Fracture Xf = 380' t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' 9

10 Field Example of Flow Characteristics 100 East Texas Example 2 to 1 Rectangular Boundary at 1,017 Days 10 Kh = 3.3 md-ft Inifite Acting Pseudo Radial Flow Boundary Dominated Flow P wd or P wd ' 1 Uniform Flux Fracture Xf = 380' t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' 10

11 Base Simulation Cases Uniform 40 acres Model Parameters Formation top, ft 10,000 Initial reservoir pressure, psi 5,000 Net pay, ft 40 Gas specific gravity 0.65 Effective Gas Perm. md 0.05 Fracture half -length, ft 200 Fracture Conductivity, md-ft Simulation Controls Flowing tubing pressure 350 psia Production time 2 years Single layer model

12 Base Case Radial Flow Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years Match Simulation Kh = 2.0 md-ft, 2.0 md-ft X f = 165 ft, 165 ft Area= 40 Acres, 40 Acres P wd or P wd ' Actual PwD Actual PwD' Analytical PwD Analytical PwD' t DA Pressure, Psia Results match simulation Average pressure 2,630 psi after 2 years 12

13 Stress Dependent Permeability Matrix Natural Fracture 13

14 Stress Dependent Permeability Reduced flow capacity Reduction in reservoir and completion flow capacity Permeability Multiplier Flowing pressure 450 psi Change in Net Net Stress, Psi psi 14

15 Stress Dependent Permeability Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years Match Kh = 1.0 md-ft X f = 180 ft Area= 32 Acres P wd or P wd ' Actual PwD Actual PwD' Analytical PwD Analytical PwD' t DA Pressure, Psia Flow capacity is reduced, radial flow False depletion stem Thirty year effective drainage area of 34 acres 15

16 Natural Fracture Parameters Fracture spacing 30 ft Flow capacity 2.0 md-ft - Matrix = md - Natural fracture = md 16

17 Type Curve Match Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years Match Kh = 0.68 md-ft X f = 165 ft Area= 23 Acres P wd or P wd ' t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' Reduced flow capacity Linear flow Limited drainage area 17

18 Pressure Profile After Two Years No Stress dependent Permeability Stress dependent Permeability Pressure, Psia Pressure, Psia False depletion stem draining ~ 20 Acres Linear flow PA after 16 years resulted in a 40 acre

19 Field Example Stress Dependant Permeability Well Location Carbonate Flow capacity Completion North Dakota ~ 10,000 ft Natural fractured Horizontal 19

20 Field Example Production Analysis Results Log-Log Plot Rate & Pressure History Depletion stem Limited Reservoir Flow capacity Effective length 13.6 md-ft 520 ft 20

21 Pressure Build Modeling Log-Log Plot E Log-Log plot: p-p@dt=0 and derivative [psi] vs dt [hr] 21

22 Actual Pressure Build Up Log-Log Plot 100 Hour Test Stimulated well performance No Boundaries 22

23 Actual Pressure Build Up Pressure Match 23

24 24 Radial Composite

25 Radial Composite Two regions considered - Inner region 5 acres, 2 md-ft - Outer region 35 acres, 0.02 md-ft Results - Reduced effective drainage area - PA match shows linear flow - Long-term complex transient behavior - PA after 25 years results show 40 acres 25

26 Type Curve Match Infinite Conductivity Fracture in 3 to 1 Rectangular Boundary at 2 years Case 3 Kh = 2.0 md-ft X f = 165 ft Area= 7 Acres P wd or P wd ' t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' Linear flow with limited drainage area 26

27 Pressure Profile After Two Years Pressure, Psia Blue area is the 5 acre higher flow capacity area Radial shape reflected in the model 27

28 Two Non-Communicating Layers Description Top layer 466 ft x 1320 ft, ~ 14 acres Length to width aspect ratio of 2.8 Bottom layer 40 acre Flow capacity for each layer - 2 md-ft - 20 md-ft 28

29 High Flow Capacity Channel Low Flow Capacity Channel Infinite Conductivity Fracture in 2.8 to 1 Rectangular Boundary at 2 years Case 4 Kh = 16 md-ft X f = 125 ft Area= 28 Acres Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years Case 4 Kh = 20 md-ft X f = 125 ft Area= 28 Acres P wd or P wd ' 1 P wd or P wd ' Actual PwD Actual PwD' Analytical PwD Analytical PwD' Actual PwD Actual PwD' Analytical PwD Analytical PwD' t DA t DA High permeability layer dominates geometry Gas in place matches the actual volume Single layer model can not capture the complex geometry 29

30 Pressure Profile After Two Years High Permeability Channel Pressure, Psia Both layers drained Linear flow geometry from PA 30

31 Hydraulic Fracture Clean-up Infinite Conductivity Fracture in 1 to 1 Rectangular Boundary at 2 years Match Simulation Kh = 2.0 md-ft, 2.0 md-ft X f = 37 ft, 165 ft Area= 40 Acres, 40 Acres P wd or P wd ' t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' Initial fracture conductivity set at 2 md-ft 60 day clean-up to a final fracture conductivity of 75 md-ft Result show short effective length 31

32 Fracture Conductivity Reducing Infinite Conductivity Fracture in 2.2 to 1 Rectangular Boundary at 2 years Match Simulation Kh = 1.8 md-ft, 2.0 md-ft X f = 181 ft, 165 ft Area= 17 Acres, 40 Acres P wd or P wd ' Actual PwD Actual PwD' Analytical PwD Analytical PwD' t DA Pressure, Psia Fracture conductivity reduced by 1% each day for the two years False depletion stem and linear flow 32

33 Liquid Loading What s The Problem? Additional back pressure on formation? Poor estimate of actual bottomhole pressure from surface data? Imbibition of water into the formation while the well is flowing and static? Will the well improve if unloaded? Do loaded wells result in a false depletion stem and reservoir shape? 33

34 34 Wellbore Dynamics - Loading

35 35 Wyoming Field Example

36 Field Example Production Analysis Match Kh = 20 md-ft Skin = -1.1 Area = 190 Acres Radial Flow in 1 to 1 Rectangular Boundary Pre-loading P wd or P wd 'x t DA Actual PwD Actual PwD' Analytical PwD Analytical PwD' 36

37 Imbibition Under Flowing Conditions Laboratory work by Stim-Lab 37

38 Liquid Loading Late Time SLC Two Year Pressure Profile Pressure, Psia Standing liquid promotes near-well damage through spontaneous imbibition. In field applications it is common to see both linear flow and false depletion stems Pressure profile from the model confirms the false depletion 38

39 East Texas Field Example Sims Gas Unit No. 1 East Texas Well Gas Rate & Tubing Pressure, (mscf/d, psi) MCFD FTP BWPD Installed Pumping Unit Water Rate, bbl/d 0 8/1/2004 8/1/2005 8/1/

40 West Texas Field Example West Texas Well 40

41 Observations Complex flow conditions can cause PA to incorrectly predict flow geometry and drainage area. Actual reservoir properties can be reproduced through PA when the reservoir and the fracture are producing at a pseudo steady state conditions. When these conditions are not achieved, PA can not be expected to provide unique solutions. The cases presented highlight the need to incorporate all available data into the analysis of the well s performance and recognize the limitations of the technique being used to analyze well performance. 41

42 42 Questions?

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