Analysis of microseismicity during thermally-assisted gas oil gravity drainage in a heavy oil field in the Sultanate of Oman

Abstract

The objective of this thesis is to evaluate the use of microseismic monitoring to assess Thermally-Assisted Gas-Oil Gravity Drainage (TA-GOGD) in an oilfield in the Sultanate of Oman. The reservoir units are carbonate rocks of high porosity but low matrix permeability, and the field contains heavy oil. Pilot tests proved that heating the oil decreases its viscosity, increasing the recovery factor through the process of TA-GOGD, and lead to field-scale use of this Enhanced Oil Recovery (EOR) method. However, the field is shallow, and there have
been growing concerns about felt seismicity and surface deformation. A microseismic monitoring program detected roughly 7200 events between April 2011 and August 2015. A processing workflow is established to analyze the data, which includes determining sensor orientation, noise filtering, automatic travel time picking, 3D velocity model building, and event location and characterization. Located events cluster around two major fault systems, oriented NW-SE and NE-SW. The induced microseismic events are related to fracture initiation around these faults, reflecting the local stress state. Events locations are concentrated in the reservoir units where the steam injection is taking place, and there is no evidence of seismicity into the overburden. This means that steam is not leaking into the shallower layers, and there is little pressure buildup in the caprock. The seismicity also reveals an active fault that was not detected in seismic reflection surveys. Moment magnitude varies from -0.18 to -3.12 with an average of -1.75. Estimated b-values for different event clusters range from 1.5-2.2, indicating swarms of fluid-induced low magnitude events and the diffusion of fluids into fractures. Calculated d-values for each event cluster are between 1 and 2, implying that they are mostly clustering in a planner manner (fault plane), or related to steam flow. Microseismic monitoring of TA-GOGD shows the caprock and fracture network system’s integrity during prolonged exposure to steam injection.

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	James P Verdon (Supervisor) & John Michael Kendall (Supervisor)