Unlocking Hidden Reservoir Potential Through Integrated Formation Damage Evaluation

Abstract

Formation damage during drilling and completion has often led to poor well test results and, in many cases, the wrong conclusions have been drawn. Poor productivity has been mistaken for poor permeability and connectivity. Reservoir potential has been hidden and assets abandoned. However, the recognition of formation damage in unwanted assets has led to the re-evaluation of legacy drilling and production data. Two examples are presented which demonstrate the methodologies and techniques employed in an integrated solution for the recognition, diagnosis and mitigation of formation damage. In particular, return permeability tests on core in the laboratory are essential but must be coupled with a more rigorous validation of well test interpretations before reaching any decision on relinquishment or abandonment. An integrated petrophysical, geomechanical and formation evaluation strategy can open up significant development opportunities in what were regarded as uneconomic oil and gas fields. In one case, the methodology resulted in a ten-fold increase in absolute well flow potential and the commercial development of the field. The methodologies and workflows developed in these field examples mean that it is possible to drill and develop low permeability reservoirs economically without causing damage and avoiding the need for expensive stimulation through careful planning and design. Introduction Formation damage can be defined as any reduction in near wellbore permeability as a result of drilling, completion, production, injection, attempted stimulation or any other well intervention. The total cost to the industry in deferred production and remedial damage treatments is difficult to quantify. However if one were to extrapolate from the examples described in this paper, revenues worth billions of dollars are potentially overlooked due to poor characterisation of formation damage. Certainly, efforts continue to be made to minimise formation damage in development wells and formation damage mitigation is a key element in many field development plans. Historically attention has often been more focused on microscopic formation damage mechanisms and macroscopic production enhancement technology rather than on the effective management of the drilling and completion processes to mitigate skin. As a consequence, formation damage and its impact on well productivity in exploration and appraisal wells is often overlooked, so potentially economic and prolific fields have been condemned to be non-viable as a result of poor well productivity rather than poor permeability or connectivity. Formation damage during drilling and completion has led to poor test results and, especially in an environment where reservoirs are expected to be tight, the wrong conclusions have been drawn. An integrated re-evaluation of these assets coupled with at recognition of formation damage can unlock hidden reservoir potential. Byrne et al (2007) suggested that if one could understand clearly the degree and nature of any exploration or appraisal well damage then surely this would enable better understanding of all data produced. A dedicated formation damage study during the exploration and appraisal stage becomes a tool that can be used to aid decision-making later on in field life and potentially for the development of offset or analogue reservoirs. We are often faced with the question: is this a bad reservoir or is this a damaged well? The technology which enables this new interpretation is well established, but the application of the various techniques as an exploration tool is new. The methodology outlined and the processes described have many applications in suitable undeveloped discoveries.

Exploration and development in the Carboniferous of the Southern North Sea: a 30-year retrospective

“…Salt-saturated muds were the inevitable result of leaving openhole sections that had exposed Rotliegend and / or Zechstein salts between the deepest casing point and the drilled Carboniferous section. In at least some cases this resulted from the enforced choice of casing points at depths shallower than had been planned following well control incidents in the Zechstein (McPhee & Byrne 2009). More careful mud formulation and /or the use of oil-based fluids can largely eliminate this problem (Figure 11b).…”