Problems and research performed

Task 1 consisted of two steps of three phases:

  1. Step 1, Task 1A: blind prediction of the flowrate at the source zone located in borehole FCF 3 from which a pump test had been carried out by Nirex at Sellafield, England, and the pressure response of another 18 monitoring zones in other nearby boreholes, as shown in Fig.1a, considering the effects of coupled hydro-mechanical processes on the results;
  2. Step 1, Task 1B: calibration of the numerical model for Task 1A against measured data, see Fig.1b and 1c as two examples of the numerical modeling results;
  3. Step 2, Task 1C: Prediction of coupled hydro-mechanical responses in Sector 7 of the host rock (Borrowdale Volcanic Group (BVG)) at the Sellafield site due to a planned shaft excavation, with EDZ effects as one of the main research issues.

Seven research teams participated in Task 1A and Task 1B, with their numerical modeling approaches described in Table 1 below.

Table 1: Research teams and basic computational model features for Task 1A and 1B
Team Basic model features
ITAf DFN (1D random pipe(channel) network for in 3D space) , code 3FLO, flow only
ITAs FDM, Equivalent continuum, 2D-axisymmetric model centered around FCF3 borehole
KIPH FEM, equivalent continuum, 3D model containing faults, codes SISIM & SETRA
KTH DFN, 3D, flow only through 7 faults solved by FEM, codes FRANMAN and MAFIC
PSM DFN, 3D, coupled stress-flow process, code FRACAS
UNEW FDM, equivalent continuum. 3D stress-flow coupling, code PARADIGM
UPV FDM, fractured continuum, 3D, codes GCOSIM3D, ISIM3D, INVERTO
Figure 1: a) Nirex’s RCF (Rock Characterization Facilities) layout where the RCF3 well is located at the central area; b) accumulated percentages of monitoring zones with pressure >5.0 MPa vs. time, Task 1B (calibration); c) drawdown vs. time, Task 1B (prediction). The solid line for Nirex was the measured data.

Six teams participated in Task 1C, with their numerical modeling approaches described in Table 2. Only blind numerical predictions were performed without calibration, due to a the fact that the continuation of the RCF program at Sellafield site was halted, including the Nirex’s shaft excavation project.

Table 2: Research teams and basic computational model features for Task 1C
Team Basic model features
ITAs FDM, faulted equivalent continuum, 3D, code FLAC3D, uncoupled H- & M-processes
KIPH FEM, equivalent continuum, 3D model containing faults, code THAMES
KTH FDM, 3D, code FLAC3D, M-process only, no EDZ considered
PSM Uncoupled H- & M-process, FEM for stress analyses (code VIPLEF), DFN for fracture flow (code FRACAS)
UPV FEM, fractured continuum, 3D, codes ANSYS, GCOSIM3D, ISIM3D, and INVERTO. No EDZ
VTT Uncoupled H-& M-processes. 3D, FDM (code FLAC3D) for stress analysis, DFN (code TRINET) for fracture flow. No EDZ.

Achievements and outstanding issues

Task 1A and 1B were successfully carried out with quite a diverse combination of different modeling approaches, numerical methods and computer codes, and material characterization schemes. The large amount of site characterization/investigation information enabled more reliable numerical modeling, which led to reasonable results for modeling the pressure field and the general trend of the flowrate data, as demonstrated by a number of published journal papers. However, significant uncertainties stll existed, which related to flow properties, characterization of field conditions based on reliable understanding, and use of the site investigation data for developing more reliable modeling approaches, especially model conceptualization concerning fracture systems given the huge amount of site investigation data.

For Task 1C, only blind predictions were performed since the shaft excavation could not be carried out, so that a logical discontinuity existed between Task 1A+1B and Task 1C. However, efforts were made by the Task 1C teams, helped greatly by the Nirex representative in the DECOVALEX project, to establish possible links between Task 1B and 1C using the site investigation information available. As a result, useful modeling was conducted for understanding the effects of the coupled stress-flow processes of the BVG caused by shaft sinking, especially on the EDZ phenomenon at a time when the EDZ issue was just started to be considered more widely and comprehensively in the international community of GDRW.

The main outstanding issue of Task 1 was the need for more in-depth research on characterization of the site geology and the physical and chemical processes—as the basis for more successful and reliable numerical modeling of coupled THM or THMC processes, especially for fractured crystalline rocks, such as the BVG at Sellafield, UK.