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Technical Paper: Dipole Radial Profiling and Geomechanics for Near Wellbore Alteration Detection to Improve Productivity in a Matured Field

Society: SPE
Paper Number: 112862
Presentation Date: 2008
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Abstract

Cartojani is a mature oil field with depleted reservoir pressure, supported by an aquifer in the deeper Cretaceous horizon. The Cartojani structure is located in the central alignment of the Moesic Platform. It is a monocline with large dimensions and low layer inclinations. The main hydrocarbon accumulation is found in the Sarmatian formation (Base Cretaceous Paleorelif) at the depth of 1100 to 1150 m. Currently the main productive horizons are sands from the lower Sarmatian (Basal Sarmatian). The facies variation can be seen both vertically and horizontally on a well-to-well basis even though the wells are very closely spaced. Sands have different oil retainer capacity and flow from clean to dirtier sands. The lower most units comprise of unconsolidated sands that are thinly distributed. These unconsolidated sands are normally completed using cased hole gravel pack. In order to select optimal completions, it required both identification and estimation of the radial extent of the near-wellbore mechanical alteration that might cause near-wellbore permeability impairment. The near-wellbore alteration, characterized by radial profiling of formation shear, can be correlated with the skin effect and reservoir productivity index. Due to the nature of the formation, formation damage is expected to be one of the main challenges to counter the observed decreasing production. A new dimension has been added to sonic measurements, providing measurements in several dimensions: axial, radial, azimuthal. The radial measurements provide now the opportunity to determine whether the near wellbore experienced any alteration. The sonic data was processed to extract a very reliable compressional, shear and Stoneley. Dispersion analysis confirmed the presence of alteration to the formation. Dipole radial profiling then demonstrated that in fact a 5 inch altered zone was present in the unconsolidated sands. Using the sonic data, mechanical properties and stresses were calculated. The unconfined compressive strength (approximately 200 to 300 psi) values demonstrated that in fact it concerned very unconsolidated sand. Using the mechanical properties together with the possibilities for perforation design, detailed analysis was performed. The analysis showed that the traditional gun selection would not surpass the skin and that in fact if would require deep penetrating charges with big hole (to minimize sand production). A proper gun system was selected based on the analysis. To improve the wells productivity, deeper penetration and high shot density guns were selected as a perforating method, to pass the radial extent of near - well bore mechanical alteration. After perforation, the production test showed a 3-fold increment in production compared to the previous best producers in the field. The skin estimation based on the model compared very well to the measured values during production tests.

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