Enhancing Ultimate Recovery in Mature Carbonate Reservoirs by Managing Uncertainties in Stimulation Design
Authors: Retnanto Albertus , Ryan Abraham 
Published: March 2015

Abstract
Carbonate stimulation design constitutes many uncertainties and challenges. Reservoir heterogeneity, in addition to the formation damage, complicates the attempt to understand matrix acidizing stimulation design. Without proper reservoir characterization and modeling, stimulation treatment uncertainty cannot be predicted nor evaluated, which poses difficulties for treatment evaluation and improvement processes. In practice, complete and representative data necessary for treatment design may not available.
In a mature carbonate reservoir, the degree of uncertainties is increasing significantly. The additional uncertainties come from the different hydrocarbon recovery methods across the formation which leads to different resistance to flow for the stimulation fluid that sometimes is not taken into account in the stimulation design.
The increasing number of mature carbonates reservoirs in the world calls for a methodology to evaluate the uncertainties in mature carbonate formations that consider recovery uncertainties, pressure uncertainties, and formation damage uncertainties. As operators are developing hydrocarbon potential in more complex carbonate formations, conventional matrix acidizing may not be the best treatment to achieve good production results.
This paper will present the workflow that could optimize carbonate stimulation design, taking into account the uncertainties in mature fields. Stimulation strategy should not only be based on the knowledge of petrophysical data, but also on other key parameters that influence design. Several scenarios for those uncertainties will be simulated. The outcomes of this simulation scenario will be proposed for stimulation fluid selection and optimization.

Introduction
As the oil and gas industries become more advance, the reservoirs are getting mature and some of them are getting less efficient in term of recovery. Producing the hydrocarbon from the mature reservoir becomes more challenging and requires more technology. The maturity of the reservoir does not impact on the remaining recoverable reserve only but also the strategy to maximize the recovery. In carbonate reservoir, that covers more than 70% of the hydrocarbon reservoir in the world; mature reservoir would be related to the previous stimulation, uneven recovery, pressure depletion, and near wellbore saturation change. In some specific cases, subsidence and reservoir compaction would have a significant effect of hydrocarbon production (Susilo et al. 2003).
The majority of the hydrocarbon reserves are being produced from carbonates reservoir. Besides their massive reserves, these carbonate formations share the characteristics of being carbonates, very heterogeneous, complex and difficult to be characterized. This heterogeneity and complexity poses a great challenge in achieving effective and consistent stimulation results. The main purpose of stimulation in carbonate reservoirs is to increase the connectivity between the formation and the wellbore in the entire zone of interest. Carbonate reservoirs are routinely stimulated with acid to improve production. Matrix acidizing in carbonates provides an opportunity not only to bypass damage around the wellbore, but to also improve the near-wellbore permeability by creating large flow channels (wormholes) with acid dissolution. Acid stimulation of carbonate formations using hydrochloric (HCl) acid-based systems to improve hydrocarbon production has been a common practice since the early days of the oil and gas industry.