Fracture Network Well Test Interpretation: in thevalhall Reservoir, North Sea

Transcription

1 AIPG Marcellus Shale Hydraulic Fracturing Conference Fracture Network Well Test Interpretation: in thevalhall Reservoir, North Sea Cristian Enachescu and Steve Rogers Golder Associates Celle, Germany; Vanvouver BC

2 Overview Fracture Networks and Well Tests o 5-Minute Well Test Course Testing Conceptual Models on the Valhall Reservoir

3 Overview of Transient Tests Important source (most important?) of geometric information on fracture plumbing system Cylindrical flow and beyond Dimensions, boundaries, and reading derivative curves

4 The Semi-log Straight Line Semi-Log Straight Line Plots P/ log(t) Intent to make semi-line clear Effect is a very powerful tool to interpret geometry from tests Derivative is a map of transmissivity versus distance from the well Shape of derivative constrains network geometry log Pressure Pressure Log Time log Time

5 Boundary and Dimension Effects 1-D 2-D 3-D Reservoir geometry Network/Flow geometry

6 Dimension Information from Well Tests 4.00E E E+00 2 n qr p ( r, t ) = Γ v u n n Kh (, ) / 2 3 4π ν = 1 n / 2 Dimensionless Pressure 1.00E E E E E E E E E E E E E E E+00 Dimensioness Time

7 Well Test Response Log Pressure Change and (Semi-log) Derivative Near Well Reservoir Boundaries Pressure (Semi-log) Derivative Log Time Early Mid Late

8 The Red Dot 0.1 hours

9 0.2 hours

10 0.3 hours

11 0.5 hours

12 1 hour

13 2 hours

14 3 hours

15 5 hours

16 10 hours

17 20 hours

18 30 hours

19 50 hours

20 100 hours

21 Normalized Derivative Plot and Hydrostructural Models Normalize (divide) all derivative curves by rate: rate normalized derivative Plot on a common graph Compare different wells: o o o Connectivity Properties Boundaries Example: Swedish Hard Rock Laboratory

22 High T Bounding Zones at about 100-m TRUE Hydrostructural Model

23 Identifying Connected Networks Rate-Normalized Derivatives -- Pre-test and Phase A 1.E+09 A1:KI0025F03 #20 A2: KI0025F03 #21 A5: KI0025F03 #20 1.E+08 A4: KI0023B, #21/#20 PT-1: KI0023B, #13 Normalized Buildup Pressure, Pa/m 3 /s 1.E+07 1.E+06 Structure #21 Source Structure #20 Sources Structure #13 Source PT-3: KI0025F02, #20 1/2 Slope (Linear Flow) 1.E+05-1/2 Slope (Spherical Flow) 1.E+04 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 Buildup Time, hours

24 Valhall Field

25 Conceptual Model The numerical modeling is intended to test one concept: o o flow is dominated by well connected seismic faults; sub-seismic faults and intergranular permeability are treated as matrix.

26 Fault Set Assignment RED - preferential flow pathways, typically NW-SE Yellow - no preferential flow, typically NE-SW Green - faults abutting against neighbours, no preferential flow

27 Fred Model Extend faults into 3d Add borehole trajectories Add layer surfaces

28 Dynamic Flow Simulation - Meshing Fractures from FRED meshed using FredMesh Fractures discretised into planer triangles Matrix represented as planes (this phase), or as fully discretised tetrahedral elements

29 Valhall Well Test Groups

30 Modelled Well Tests - Group 1 Borehole A13b taken as typical Match to 1987 data using default parameters! (A13b_1) Note that fault permeability x thickness reduces with time, by around a factor of 4 in 7 years (A13b_9) 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Match to 1994 data - permeability reduced from 1000 md m to 250 md m Match to 1987 data 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 ELT [h] or Distance [m]

31 Modelled Well Tests - Group 2 Borehole A4a-st1 taken as typical Reasonable match to shape and value of derivative (late time data are variable) Parameters consistent with Group 1 match Data from A4a-st1 in pink

32 Modelled Well Tests - Group 3 1.E+00 1.E+01 Mobility-Thickness Product [md ft / cp] 1.E+02 1.E+03 1.E+04 Well-bore storage Modelled derivative when fault intersects well (A8_15) Borehole A8 taken as typical Not possible to match late-time derivative shape using homogeneous, isotropic matrix Reasonable match if a fault intersects the well-bore 1.E+05 Modelled derivative for matrix+seismic faults simulation (A8_10) 1.E+06 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 ELT [h] or Distance [m]

33 Modelled Well Tests - Group 4 Group 4 responses show derivative gradient of between 0.25 and 0.5, indicative of fracture response However most of the wells with data are on the edge of the study area or are outside it Consequently no detailed attempt to match data at this stage (a generic fault model would match well) Mobility-Thickness Product [md ft / cp] 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 ELT [h] or Distance [m]

34 Lineaments and Reservoir Pressure (well A13b) After hours, pressure clearly diffusing along lineaments (permeability md) [A13b_L2]

35 Another view, without matrix

36 Conclusions from Flow Simulations The well connected seismic faults conceptual model is capable of reproducing the main well test response features, providing the near-well properties are adjusted. This is a comparatively high storage flow system, so the response to short term well tests is dominated by one or more faults that intersect or are close to the well Properties of distant faults are largely insensitive to model calibration to existing well tests.