Mini-DST Application For Thin Layer Reservoir In South Oman
Authors: Mirza Muhammad , Al Saadi Hassan , Shihab Muhamad Rizvi , Al Wardi Abdullah , Fakhrur Razi Muhammad , Anwar Zul , Zaggas John , Jumaat Mohd Shafie , Kusumadjaja Aming 
Published: May 2016

Appraisal campaigns for heavy-oil characteristic wells are a continuous challenge in today's operations. Data acquisition in such environments requires reservoir information of the highest quality before expensive development plans can be put in place. New technology, real time monitoring and integrated reservoir data are essential to understand such reservoirs. Another challenge presented by thin reservoirs is the presence of vertical heterogeneity and varying layer flow properties.

Wireline formation testers have been commonly used to acquire formation pressures pressure and reservoir fluid samples for a number of decades. Many hardware technologies and interpretation methods have been developed to acquire better quality reservoir information. Dual packer wireline formation testers offer an alternative an additional way to selectively straddle a section of a reservoir and provide the capability to conduct controlled local production and interference as well as to enable the capture of reservoir fluids. Formation permeability, anisotropy, skin factor, vertical connectivity and zonal productivity index are additional reservoir information that can be obtained from a mini-Drill Stem Test (mini-DST).

Pressure transient analysis of a mini-DST data however in such reservoirs is challenging due to the associated uncertainties such as layer flow compartments and flowing fluid viscosity. This paper discusses the use of the mini-DST method for Well-A, compared to standard production test used for Well-B. The use of integrated reservoir information obtained from Downhole Fluid Analyzers (DFA) to minimize the uncertainties in the mini-DST method is also discussed. A pressure transient analysis method for mini-DSTs is also introduced.

Reservoir parameters obtained from mini-DSTs in thin reservoir are then compared with other available static and dynamic reservoir information such as petrophysical data, well tests and production logs in order to obtain accurate interpretation results of highest consistency.

Field example will be discussed which show that smaller scale pressure transient tests often have an advantage over full scale well tests testing in terms of providing detailed layer flow behavior, vertical connectivity and flow potential in thin reservoir environments. It will also be noted that the radius of investigation of a mini-DST is limited, typically within tens of feet.

This paper demonstrates using field example that reservoir parameters can be detected when the test objective is achieved. In addition, the understanding of limitations and advantages will allow the proper selection of test types in order to meet specific objectives and maximize the full potential use of acquired data for field development plans in thin reservoir environments.

Introduction

The wireline formation tester technology consists of producing the reservoir fluid directly in the wellbore using a downhole pump so as to avoid hydrocarbons flow at the surface. After this short production period, a pressure build-up occurs. Pressure is monitored during the production and subsequent shut-in period. The result is a short DST test hence the name (mini-DST) and is usually used for reservoir characterization.

The volume of the fluid produced induces a pressure transient that extends to a certain distance from the wellbore. When the pressure transient travels beyond the damaged zone and intercepts the upper and lower boundaries of the formation, a radial flow develops and the analysis of the pressure derivative can provide the average effective permeability of the reservoir.