Constraining Reservoir Models by Integrating Data Across Disciplines¬ – A Case Study of Thin-Bedded Turbidite Field in the North Sea
Authors: Choudhary Manish Kumar , Keshri Amit Kumar , Mohan Kamalraj 
Published: May 2016

Development study can be challenging when limited data is available, but cross-discipline interaction can greatly help to improve the understanding of the reservoir architecture.

A small undeveloped North Sea turbidite field, discovered in the 1980s, was recently revisited as part of a feasibility study. Two historical wells suggested the existence of thin sand beds three to five feet thick, however seismic attribute maps were inconclusive in distinguishing whether the sands were deposited as deep marine slope feeder channels or as turbidite fans. Geochemical and pressure data differences from the wells indicated a possibility of compartmentalization. The NtG estimated from the wells ranged between 10–30%, and as such large uncertainties exist in the reservoir properties and lateral extent.

Data available across all disciplines was integrated to define a range of possible Mutually Exclusive and Collectively Exhaustive field conceptual models. These were created by combining deep marine channels and turbidite lobe reservoir geometries. Sector models for both geometries were constructed and dynamically simulated to understand their impacts on recovery factor. The conclusions were used to calibrate the conceptual models and also to choose deterministic models representing low, mid and high case recoverable volumes.

An uncertainty of fluid-fill in the reservoir existed as the wells did not log fluid-contacts and had different RFT pressures. Multiple fluid-contact realizations were built that ranged from spill-point-based contacts to free water level. Realizations with perched water and stratigraphically trapped oil columns were constructed and then discarded, as the base reservoir required uplift which was greater than the seismic depth uncertainty. The realizations defined for sand distribution and fluid contacts were probabilistically combined to define the low, mid and high case realization models for the dynamic simulation. This integrated workflow helped define the worst and best case scenarios for the highly heterogeneous system with limited data.

Introduction

The introduction of a small field allowance in the United Kingdom[1] has encouraged companies to revisit previously discovered fields for potential development. Many of these small fields were discovered in the 1980's and 1990's and have limited appraisal data, resulting in large sub-surface uncertainties. This makes the development planning quite challenging as smaller volumes in these fields often result in marginal economics. At the same time however, they offer a cheap development opportunity though a tie-in to a nearby facility. Such tie-ins also help in extending the life of the existing fields and facilities, thereby increasing overall value for the company.

Shell recently completed a development feasibility study for a small field, Field X located in North Sea, for possible tie-in with a nearby facility. The field was discovered in the 1980's and large sub-surface uncertainties exist in reservoir extent, fluid contacts and reservoir properties. An integrated study was carried out to define sub-surface models that honored the available dataset and also captured the uncertainty range i.e. models being Mutually Exclusive and Collectively Exhaustive. These sub-surface models were then used in dynamic realm to understand the impact of dynamic uncertainties like aquifer strength, size of the gas cap and relative permeability with an overall objective of generating low, mid and high production forecasts.