REVIEW: What Geoscientists Need To Know About Developments
Review by Clara Abu, WesternGeco
Geologists from varying disciplines, exploration in particular, converged on the PESGB Croydon office on a bright sunny day to gain an improved understanding of successful development opportunities and techniques for assessing associated data. The course was led by Stephanie Kape of Solar Geosciences Limited.
The day started by looking at the differing scales of measured or interpreted data for appraisal and development opportunities, including wireline logs, seismic, core, structural maps, well tests, fluid data and field analogues. This was followed by an introduction to PVTs and formation pressure data and a discussion on the concepts of primary, secondary, enhanced oil recovery mechanisms and development plans.
Next we analyzed the successful case study of the Harding Field discovered in 1988 in Block 9/23b, 320km NE of Aberdeen in Tertiary massive sands of the Eocene Balder Formation. Ten horizontal wells were used to produce from the Central and South heavy oil accumulations overlain by gas cap at a standoff of 70ft from the gas oil contact. Pressure support was maintained by secondary water injectors. The field illustrates how good vertical connectivity and long-term careful management of drawdown without coning gas into the wells, maintaining reservoir pressure above bubble point and maintaining a single phase around the producers, led to high recovery rates.
We kicked off the afternoon session by evaluating reservoir architecture. The vertical and lateral arrangement of flow zones and connectivity between these zones serve as important controls on recovery and are largely determined by depositional setting and accommodation. We reviewed layered systems and the relationship between increased levels of layering and preferential water breakthrough in high permeability units. This led to the second case study, focused on the Cormorant Oilfield, located approximately 150km northeast of the Shetland Islands in Blocks 211/21a and 211/26a of the UK northern North Sea. The field produced from Middle Jurassic deltaic-shallow marine Brent Group sands and was subsequently appraised with eight wells. The wells produced at a high initial rate but the heterogeneity and compartmentalization of the reservoir by sealing faults led to high water cut and an uneven flood front. Pressure data showed 6 different contacts, with notable decrease in porosity and permeability with depth. This field was later managed by TAQA and the conclusion drawn was that long-term productivity and incremental recovery from the field was possible through careful management of the high water cut.
We reviewed reservoir models as powerful tools for quantifying uncertainties and highlighting risks. Generating a series of deterministic cases that explore very different outcomes can be a more rewarding route than generating multiple probabilistic scenarios that end up being complex and inefficient. On the theme of heterogeneous reservoirs, we analyzed a third case study in channelized Triassic fluvial succession where exploration well tests had given misleading results. Steep pressure decline from initial production indicated that the reservoir was more heterogeneous than predicted, leading to a revision of the geological model. Participants finished the course with a firm understanding that integration of geological concepts into engineering models was key to successful development of the life of the field.
We extend our thanks to Stephanie Kape for a very insightful one-day course and recommend the course for geoscientists who have a keen interest in field development.