Efficient Evaluation for an exploratory well with multiple targets is the key to achieve the exploration objectives and success. For that, define Test Strategy has become a critical issue for most of the operators. However, it has not received yet attention in the literature.
The main goal of this paper is to formulate a discussion for well-testing strategy and management approach that meets exploratory evaluation objectives in the most effective and economical way. It can be achieved through good understanding of the following.
Exploratory well: aim of evaluation through well testing
Well test design and implementation
Testing method: economic evaluation
Perforating and testing management
DST rig operations versus rig-less testing evaluation
Proper weighting of the previously mentioned points will help to decide the suitable testing technique; retrievable DST string or permanent completion testing string, in other words Rig or Rig less operations. Reservoirs of high uncertainties on the expected response require flexible techniques that can handle any unexpected conditions. They are thus candidate for retrievable DST string rig operation. Otherwise reservoirs of lower uncertainties are usually candidate for permanent completion to test on rig less operations without DHSI (Down Hole Shut-in Tool).
The majors benefits of the permanent completion testing string over the retrievable DST testing operation are the flexibility in test time and that the rig could be free to drill new wells, mainly in the running times where is very difficult to find available rigs. Nevertheless, one need to weight Time spent versus Value before making a final decision. Few cases from Repsol Exploration SA wells will be presented and discussed.
Repsol is exploring inside the North of Africa (see map in Figure 1). The age of the formations targets are: Carboniferous, Devonian, and Cambro-Ordovician, these targets are found at the depth ranging between 2000 to 5000 mbrt; normally they are tight gas sandstone reservoirs.
Based on the testing results, from all the tested wells in the area of the contractual permit, the produced hydrocarbon was dry gas, and the formations are of low permeability.
Until now, in the exploratory campaign the wells were drilled and tested in rig and rigless operations.