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Article Type

Review Article

Highlights

  • Formation compressibility was estimated by applying fluid substitution based on Gassmann equation.
  • Simultaneous seismic inversion allowed P-wave/S-wave velocities and density calculations which then were used to estimate formation compressibility at reservoir level.
  • The methodology enables to characterize the spatial distribution of formation compressibility which may be useful as an input data in numerical reservoir simulation.
  • Obtaining formation compressibility in low-shale content sand zones reduces the deviation from Gasmann equation assumptions, making more reliable its application.

Abstract

In shallow siliciclastic reservoirs, above 1,000 meters, composed of unconsolidated sands and high oil viscosities, the compressibility of formation plays an important role as a driving mechanism that contributes to the expulsion of hydrocarbons and the improvement of flow during production. Thus, the objective of this work pursuits the development of a methodology to calculate this formation property and its distribution throughout the reservoir. This knowledge could lead to find suitable areas for higher oil production rates. A methodology was implemented for one petroleum field locating in the southern Gulf of Mexico based on simultaneous seismic inversion and fluid substitution applying Gassmann equation to estimate formation bulk volume and subsequently formation compressibility in unconsolidated sand reservoirs. The oil reservoir under study has unconsolidated sands with a depth range from 700 to 1,000 meters, with porosities and permeabilities between 25 - 35% and 100 - 10,000 mD respectively. The average thickness of sands is around 14 to 18 meters and areas of 0.5 to 1 km2. The estimate was made in five wells which had core mineralogic information, acoustic logs and fluid properties, then using seismic reflection data, it was extended to reservoir level. The results are similar to those formation compressibility values reported in literature related to unconsolidated sands which validates the proposed methodology consistency for its application. Furthermore, when it is applied at reservoir level, it enables to obtain this formation property for a more accurate reservoir dynamic characterization, in particular for numerical simulation that has the particularity to be the only approach to consider the spatial distribution of this parameter. It is convenient to point out that this methodology must be applied only on undersaturated oil reservoirs (absence of a gaseous phase). Since the magnitude of gas compressibility is much bigger compared to the other formation phases (liquid and solid), its presence could mask the response associated with liquid saturation and rock volume making invalid some Gassmann equation assumptions.

Keywords

Formation Compressibility, Fluid substitution, Seismic inversion, Gassmann´s equation, Bulk modulus, Elastic Bounds

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