An important premise of underbalanced drilling (UBD) is the productivity improvement it delivers through mitigation of invasive damage. Characterization and quantification of such damage, therefore, becomes a pre-requisite for assessing the value delivered by UBD. Several methods are available to quantify damage. In this work, we use a novel approach that combines dynamic micro-scale reservoir simulations calibrated to special core tests to model the extent of invasive damage, and its impact on flow-back during production. The approach is based on special tests conducted on reservoir core, and a dynamic "micro-simulator" to model invasion during drilling. Special core tests designed to measure effects of overbalanced exposure to drilling fluids are first conducted. Inputs to the simulation model are based on careful interpretation of the core test results, and thus are calibrated to observation. Details of the approach were presented earlier by Suryanarayana, et. al. (SPE 95861). In this paper, we apply this approach to two field cases, and use the results to quantify the damage and its impact on production. The two field cases are discussed in detail. Both relative permeability and permanent damage effects are described. The dramatic effects of invasion on clean-up and long-term production are illustrated, thus demonstrating the incremental value of UBD in these cases. Damage can be modelled as an equivalent skin, based on the saturation and permeability profiles within the zone of invasion. Since the saturation and permeability effects are a function of location along the productive length as well as time, we obtain time- and spatially-dependent equivalent skin. The equivalent skin can then be used in field-scale reservoir models to compare different drilling and development options. The use of these results in designing an optimal drilling and completion plan to lock-in the value of UBD is demonstrated for the two field cases.
Three distinct advantages of underbalanced drilling (UBD) technology can combine to lower the unit technical cost of a project:
Reduction in overbalanced drilling problems,
Reduced formation damage, and
Dynamic reservoir evaluation while drilling.
However, in low-cost drilling environments, such as land operations in the Middle East and in North America, drilling-enabling savings from UBD are often marginal and the cost of UBD operations becomes a blocker for wider implementation, as the promise of production enhancement and dynamic reservoir characterization are not properly quantified. Implementation of UBD in Russia, Asia and other low cost areas will face similar hurdles. The UBD implementation dilemma is that while we need to prove value in order to move the technology forward, we also need data that demonstrate value. Analogue information can be used to develop the business case, but there is a limited data set due to perceived high cost of UBD and questionable effectiveness of the technology in the candidate reservoir. Implementation costs are driven by utilization of the equipment and low utilization is driven by a lack of candidate wells. Even after a successful trial, additional candidates require cost benefits of commoditization of the technology but commoditization requires widespread uptake of the technology and uptake requires the recognition of the value. The accepted practice for executing a "green" field development plan is to use a full field dynamic model to determine potential value and use as input into the final investment decision. However, modelling is often based on unproved initial assumptions compounded by the lack of UBD well performance data; and the cycle of uncertainty repeats. This traps UBD implementation in a " Catch 22″ cycle, (illustrated in Figure 1) which we believe is one reason for its limited uptake in some areas. In recent times, different approaches have emerged to better quantify UBD value and break out of this cycle.