Compaction drive reservoirs

If pore volume contraction contributes prominently to overall expansion while the reservoir is saturated, then the reservoir is classified as a compaction drive. Compaction drive oil reservoirs are supplemented by solution gas drive if the reservoir falls below the bubblepoint; they may or may not be supplemented by a water or gas cap drive. This article discusses characteristics, performance and material balance of compaction drive reservoirs.

Characteristics of compaction drive reservoirs

Compaction drives characteristically exhibit elevated rock compressibilities, often 10 to 50 times greater than normal. Rock compressibility is called pore volume (PV), or pore, compressibility and is expressed in units of PV change per unit PV per unit pressure change. Rock compressibility is a function of pressure. Normal compressibilities range from 3 to 8 × 10^–6 psi^–1 at pressures greater than approximately 1,000 psia. In contrast, elevated rock compressibilities can reach as high as 150 × 10^–6 psi^–1 or higher at comparable pressures.

In general, compaction drive reservoirs are rare; however, strong compaction drives do exist. The Ekofisk field in the Norwegian sector of the North Sea, with reserves in excess of 1.7 billion bbl, lies at a depth of 9,300 ft below sea level in 235 ft of water. The reservoir is a chalk formation that exhibits porosities in the range of 25 to 48%. The operators reported rock compressibilities as high as 50 to 100 × 10^–6 psi^–1. Extreme compressibilities such as these can account for 70 to 80% of the expansion above the bubblepoint and 20 to 50% or more of the expansion below the bubblepoint.

Performance

Compaction drive oil reservoirs act like their noncompaction counterparts except that they exhibit enhanced recoveries. For instance, a solution gas drive, compaction drive reservoir will act qualitatively like a normal solution gas drive reservoir except the oil recovery will be greater. The enhanced recoveries are a direct consequence of the extra rock expansion that compaction drive reservoirs naturally possess.

The excessive compaction noted in compaction drive reservoirs has contributed to some production problems. For example, the compaction has been linked to a decline in reservoir permeability, fracture closure, and subsidence. In most cases, however, these problems have been manageable, and the net result of compaction has been very favorable.

Material balance analysis

The material balance methods are equally applicable to compaction drives. The only difference is that rock expansion cannot be ignored. Including rock expansion requires evaluating the rock expansivity, E_f.

The most accurate and reliable method is to measure E_f as a function of pressure. This method is strongly recommended if a compaction drive mechanism is suspected because of the sensitivity of the analysis to E_f. Table 1 summarizes the experimental results for a high pressure Gulf Coast gas reservoir. The initial reservoir and hydrostatic pressure was 9,800 psia. The rock expansivity ranged between 0 and 8.07%. The porosity decreased from 16.7% to 15.5% over the course of the test. This particular sample exhibited higher than normal expansion.

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  Table 1 - Experimental rock compressibility results for a high pressure Gulf Coast reservoir.

The rock expansivity and compressibility are related through

....................(1)

where c_f is the rock compressibility. This equation assumes that the fractional change in PV is small. Physically, the rock expansivity represents the fractional change in PV while, in contrast, the rock compressibility represents the rate of change in fractional PV with pressure. While the former is more pertinent to material-balance calculation, experimental data are often reported in terms of the latter. Table 1 includes rock-compressibility measurements. If c_f is known as a function of pressure, then the integral on the right side of Eq.1 can be evaluated numerically to determine E_f(p). If c_f is relatively independent of pressure, then Eq.1 can be simplified to

....................(2)

This method of estimating E_f is not usually preferable because c_f is rarely constant. Fig.1 illustrates a case and plots the rock compressibility as a function of pressure from the data in Table 1. Several features are worth noting, and many of these features are characteristic of compaction drives.

• Rock compressibility ranges between 4 to 21 × 10^–6 psi^–1, which is a greater-than-normal range.
• Rock compressibility clearly is not independent of pressure.
• Compressibility declines sharply as the pressure first declines below the initial pressure. This phenomenon is largely attributed to grain rearrangement.
• Rock compressibility increases at pressures below 4,000 psia. This phenomenon is attributed to pore collapse.

• 

  Fig. 1 – Rock compressibility as a function of pressure for a Gulf Coast reservoir.

Once E_f(p) is estimated, the material-balance methods in the related pages below can be applied to estimate the OOIP and confirm the producing mechanism.

Nomenclature

cf	=	rock compressibility, Lt2/m, 1/psi
Ef	=	rock (formation) expansivity
p	=	pressure, m/Lt2, psi

Noteworthy papers in OnePetro

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See also

Material balance in oil reservoirs

Solution gas drive reservoirs

Gas cap drive reservoirs

Water drive reservoirs

Oil fluid characteristics

PEH:Oil_Reservoir_Primary_Drive_Mechanisms