Complex Reservoir Architecture Validated by Integrating Well Testing Outcomes
Authors: Azzarone Eleonora , Beretta Enzo , Salvadori Sergio , Bettazzoli Pietro 
Published: May 2016

This work presents a real field example where the integration of well testing and geological interpretations increased the understanding of a complex and heterogeneous reservoir.

An integrated study of both dynamic well testing data and geological/seismic models was carried out to assess a very complex pressure response obtained from a production test conducted in an offshore oil well.

The well testing objectives were both to improve reservoir characterisation and to quantify the well productivity in a high permeability oil bearing sandstone formation.

In particular, the test was aimed at verifying any presence of heterogeneities and/or permeability barriers within the radius of investigation to compare with the current geological/seismic model.

The testing operations were performed on a deviated appraisal well, located in a deep water offshore field. The well had been previously completed using sand control technology. The bottomhole pressure and temperature data were acquired with wireless transmission technology, allowing data acquisition in real time. Tidal effects were identified and removed from the pressure response.

Different overlapping sand bodies, previously intercepted by a nearby well, contributed to the complex geological/sedimentological architecture.

The significant variations in the permeability thickness product, observed in the middle time pressure response, were found to be consistent with the geological model. The information from the late time effects was also integrated, supporting the overlapping sand body model.

Introduction

The reservoir object (named Middle Sequence) selected as the target of the production test (DST) discussed in this paper is considered one of the most important for the development of a field asset, located in ultra deep water. In fact, both the dimensions involved and the average petrophysical properties are expected to sustain quite good production performances, better than the other levels, shallower in the Upper Sequence and deeper in the Lower Sequence, included within the development plan.

A dedicated test giving good and reliable interpreted results was considered essential to assess the expectations on the reservoir productivity and extent, thus confirming the viability of the development project. In particular the optimal combination of relevant STOIP and sand reservoir quality of this object were deemed the main requirements to guarantee oil rates in the order of 20,000 STbopd per well thus playing a major role in achieving the challenging field oil plateau rate planned to be delivered to the designed FPSO. It should be remembered that the only well test performed over the field in the past, on Well 2, was inconclusive due to sanding (no well test completion with sand control was adopted).

The current production testing operations were thus conducted on the appraisal Well 3 on a drilling ship location (the water depth is approximately 6,200 ft), with the aim of reservoir characterization and productivity assessment of the oil bearing formation which is the deepest level (namely β Layer) of the Middle Sequence reservoir and constists of Miocene channelized deep water turbidite sandstones. The reservoir hydrocarbon is a light oil fluid, with a measured density of 42 °API.

The DST addressed the general main well testing objectives: the acquisition of representative reservoir fluid samples (at surface and downhole), the estimate of formation pressure and temperature, average permeability, well damage, the assessment of the well productivity and the detection of late time effects, i.e. any presence of heterogeneities and/or permeability barriers within the radius investigated by the test with respect to the current geological/seismic environment.