Summary
Detailed structural interpretation has been performed to support development of tight and thin accumulations in the Al Shaheen Field, offshore Qatar. The structural modeling has been improved by integration of new high resolution seismic data and long horizontal well data from the Cretaceous reservoir sequence.
Seismic evaluation has shown a relationship between Permo-Triassic structural elements and subtle structural elements in the Cretaceous section. Horizontal well data has provided structural modeling details beyond seismic resolution linking seismic and reservoir observations to reservoir flow unit scale. The integration of the two data sets has resulted in an improved predictive model of fault zones. Individual fault vertical throws of less than 10 feet have been observed associated with natural fracture systems and topographic features such as abundant local circular depressions.
The integration of seismic and well data has allowed building a detailed predictive model providing key input to the challenging development of the thin and tight Cretaceous reservoirs in the Al Shaheen field.
Introduction
The dominant structural features in the Al Shaheen field comprise a conjugate fault system of E-W main strike direction and a less pronounced NNW-SSE striking component. Parts of the field further display well developed northward down-stepping fault blocks. The vertical throws on the faults in the Cretaceous section are generally less than 10 ft, not directly resolvable in seismic. The seismic structural interpretation was carried out with emphasis on faulting in the homogenous Kharaib B carbonate reservoir to allow integration with well data.
The development of the Al Shaheen field includes more than 200 wells positioned in a tight well spacing with ultra long horizontal wells, including world record well length of 40,320 ft, logged with electric petrophysical evaluation tools. The majority of the horizontal wells are drilled in the Cretaceous Kharaib B reservoir comprising platform carbonates. The reservoir characteristics are particularly suitable for detailed structural interpretation from well data as a result of field wide lateral correlation of thin stratigraphic units.
Formation Integrity Tests (FIT) or Leak-Off Tests (LOT) are common techniques to reduce the uncertainty in Fracture Gradient (FG) prediction for well planning, but are usually performed at the casing shoe. This article will discuss the first examples of open-hole LOT and FIT in Petroleum Development Oman (PDO), targeting depleted formations in water injector or oil producer wells. The data was used to justify continued drilling of slim wells with two casing strings, where otherwise three casing strings would be required, provided dynamic wellbore strengthening is applied.
In addition, the concept of static wellbore strengthening was also trialed for the first time in Oman, using the hesitation squeeze testing procedure, by which the effective leak-off pressure was incrementally increased to match the maximum ECD required for cementing.
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