Formation Evaluation Society of Malaysia (FESM)

Formation Evaluation Society of Malaysia (FESM)

Development of Shaly Sand Reservoirs in Mature Fields

The shaly sand reservoirs of SE Asia have been well known as a challenging environment for field development for many decades.  The combined effects of detrital shales and authigenic clays along with brackish and variable salinity formation waters renders many of our reservoirs as low resistivity contrast pay.  Equally problematic are the large number of laminated shaly sand reservoirs with low resistivity pay.  These phenomena have not only been the cause of initial underestimations of the saturation of hydrocarbon, but have also made fluid typing extremely difficult in some cases.   

Our improved understanding of these reservoirs has resulted in substantial increases in in-place-volumes of hydrocarbon and the realization of reduced initial recovery factors in recent years.  This in turn has led to significant opportunities for re-development in many fields.  A new wave of secondary and tertiary recovery projects in the initial reservoirs, as well as infill drilling for remaining oil and gas in marginal and newly identified reservoirs has been initiated.

Most primary field developments had the great advantage of static reservoir conditions.  Fluid and pressure distribution behaved according to capillary theory and buoyancy within relatively large hydrocarbon systems.  Initial field development plans were mostly concerned with defining the static gross reservoir volume of only the best reservoirs, mostly with seismic, and log and core data from a small number of appraisal wells.

However, as experience shows, primary production during this initial harvesting phase did not exactly follow average reservoir properties as planned.  Rather, the fluids in the perforated reservoirs took the paths of least resistance, leaving a very complex fluid distribution both vertically and laterally.  Some reservoirs dominated production profiles and incurred large pressure drops resulting in phase changes, some remained virtually undepleted along with significant changes in fluid contacts, while others remained virtually untapped for a variety of reasons (LRLC, flank rims, attic volumes, misunderstood fluid types, etc.) 

The challenges for this new phase of development are much greater.  Now there is a need to understand the reservoirs in greater granularity and in more dimensions.  This includes a more detailed understanding of their stratigraphic distribution and continuity, current fluid types and pressures, hydrocarbon displacement efficiencies, and stress regimes. 

Economic success will be more challenging as well, especially in a cost constrained environment.  Most of the best reservoirs will have been pressure depleted, and have reduced hydrocarbon rates with higher GOR and WOR profiles.  Minor reservoirs, although not necessarily depleted, will generally produce smaller volumes at lower rates.  Some of previously undeveloped or unrecognized reservoirs may not be as conformable to existing platforms and may require extended reach drilling and special considerations in terms of borehole stability issues due to their more shaly nature.     

Meanwhile, there have been many improvements in subsurface technologies that can help our understanding of these reservoirs and help mitigate some of the key issues.  Technology advancements in acquisition and interpretation of Wireline/LWD/Cased Hole/Production logging; sequence stratigraphy and reservoir modelling; as well as dynamic simulation, tracer technology, drilling and completions, to name a few, can all help in this regard.     

This special topical conference is intended to share our collective multidisciplinary experiences in planning and executing these new field developments.  By increasing our understanding of the properties of these remaining hydrocarbon resources we can all reduce uncertainty and risk, maximize production, and optimize cost.