Equilibrium Contact Mixing (ECM) Module
In this module, we estimate "insitu-representative" fluid samples when they are not available from direct measurement, either because it was not possible to sample "insitu-representative" fluid samples (e.g. slightly undersaturated systems or saturated systems) or that it was just not prioritized at a point in time. This is achieved by applying different equilibrium contact mixing (ECM) procedures, in which becomes particularly handy in cases where fluid samples are only available from a for instance
- oil well experiencing gas coning
- reservoir subject to solution gas drive (producing GOR > solution GOR)
- partially depleted solution gas drive reservoir that subsequently has undergone water flooding
- depleted saturated/undersaturated gas condensate reservoir
- reservoir with saturated gas-oil contact (GOC), in which a sample is available from the oil zone and gas zone, and the composition at the GOC wants to be estimated
- well with saturation pressure higher than the intitial saturation pressure
Note
A "reservoir representative" sample is any sample produced from a reservoir. As a special case, an "insitu-representative" sample represents the volume-weighted average of original fluid(s) in the depth interval drained by a well during sampling.
ECM for Conventional Reservoirs
Collecting and Preparing Reservoir Samples
Samples of the reservoir oil and reservoir gas are first made up in the laboratory. Separator samples should always be used for the reservoir gas, and usually for the reservoir oil. Bottomhole samples can be used for the reservoir oil when available (and considered "reliable"), though separator samples are preferred if available. The reservoir gas is made by recombining separator samples to yield the actual test wellstream (i.e. using measured separator GORs in the recombination, corrected if necessary for liquid carryover, etc.). The separator samples from the gas zone should not be recombined specifically to obtain a dewpoint equal to the initial pressure at the GOC. When using separator samples to recombine a reservoir oil, the actual test GOR should be used to make the recombination, taking into account any valid corrections to the recombination GOR (oil meter corrections, gas rate corrections, etc.). An initial recombined oil sample should have a bubblepoint close to the original reservoir pressure. A recombined oil bubblepoint much less than the original reservoir pressure might indicate compositional grading with depth. A recombined oil bubblepoint much greater than the original reservoir pressure might indicate gas coning during sampling. In both cases, the proposed ECM procedures require that test GOR be used for recombination of the reservoir oil sample to obtain a true sample of the produced wellstream.
Initially Saturated Gas/Oil Reservoirs (ECM1)
Initially Saturated Gas/Oil Reservoirs (ECM1) The containers with reservoir gas and reservoir oil samples should be brought to single-phase conditions. The two samples are transferred to a PVT cell in a ratio that results in an oil volume fraction of 50% or greater at equilibrium. Slightly more accurate equilibrium compositions are obtained using higher oil fractions, but with the disadvantage that smaller reservoir gas samples are available for subsequent studies (e.g. constant volume depletion). The PVT cell is brought to initial reservoir conditions at the gas-oil contact and mixed thoroughly to establish equilibrium. The resulting equilibrium oil and equilibrium gas should provide excellent estimates of the original insitu fluids at the GOC. Each phase is removed to separate containers for further analysis. Compositions and PVT data are measured for each sample separately. This procedure represents the equilibrium contact mixing method ECM1. The ECM1 method can also be used for an initial oil well test with gas coning. The separator samples are recombined at the producing GOR measured at the time of sampling. The mixture is brought to initial conditions at the GOC and mixed thoroughly to establish equilibrium. The resulting equilibrium oil and equilibrium gas should provide excellent estimates of the original insitu fluids at the GOC. For systems with gravity-induced compositional gradients having a saturated GOC, the ECM1 method (applied at initial GOC conditions) does not provide estimates of equilibrium composition at the GOC. Instead, the method provides a depthaveraged estimate of the insitu oil between the GOC and oilsampling depth, and a depth-averaged estimate of the insitu gas between the GOC and the gas- sampling depth.
Depleted Solution Gas Drive Oil Reservoirs (ECM2)
A second ECM method (ECM2) can be applied to oil reservoirs currently being depleted by solution gas drive (producing GOR must be greater than the solution GOR at current average reservoir pressure). The method provides an estimate of original insitu oil composition, when this composition was not obtained initially. The method works for initially saturated and undersaturated oil reservoirs. The produced wellstream sample is prepared by recombining separator samples using the test GOR. The recombined mixture is brought to equilibrium at the current average reservoir pressure and temperature. All of the equilibrium gas is removed at constant pressure to another container. The equilibrium oil remains in the PVT cell. The equilibrium gas is injected incrementally back into the PVT cell containing equilibrium oil. After each injection, the bubblepoint is measured. When the bubblepoint reaches the original reservoir bubblepoint, this mixture can be considered a very good approximation of the original reservoir oil. As shown in the examples below, the resulting composition is better than simply recombining separator gas and separator oil samples in a ratio that yields the original bubblepoint pressure (particularly for intermediate components C2-C6, but also for C1 and C 7+).
Undersaturated Produced Oil Sample
Accurate estimates of original oil composition can usually be obtained if the produced reservoir oil sample is undersaturated relative to the current average reservoir pressure. This might occur during the period of critical gas saturation buildup, or in a partially depleted solution gas drive reservoir that has subsequently undergone water flooding. During the period when reservoir pressure first drops below the original bubblepoint and a critical gas saturation is building up, the produced wellstream may be undersaturated relative to the current reservoir pressure (ECM1 and ECM2 methods will not work). Recombining separator samples at a GOR that yields the original bubblepoint pressure has been found to give accurate estimates of the original reservoir oil. Many oil reservoirs have been repressurized by water flooding after previously having undergone depletion by solution gas drive (e.g. many West Texas CO2-flood reservoirs). The only practical laboratory method we have found successful for creating an approximation of the original reservoir oil is recombination of currently producing separator samples to the original bubblepoint pressure.
Depleted/Saturated Gas Condensate Reservoirs
An estimate of the original gas composition of an initially saturated gas zone (with underlying oil) can be achieved in a depleted reservoir. Separator samples must be collected from both the gas zone and the oil zone. First an ECM2 procedure is performed on the reservoir oil to create an estimate of the original reservoir oil. The resulting ECM2 oil sample is mixed with the reservoir gas sample using the ECM1 procedure. The final mixture is brought to equilibrium at the original GOC conditions, with the equilibrium gas providing an accurate estimate of the original reservoir gas.
ECM for Unconventional Reservoirs
For slightly undersaturated reservoirs, and saturated reservoirs, it may be impossible to sample in-situ representative fluid samples due to near-wellbore multiphase behavior. However, samples which are not in-situ representative can still be used to estimate original in-situ fluids either computationally using an EOS model, or by physical measurement in the laboratory.
Fevang and Whitson (1994), presented how different methods of equilibrium contact mixing (ECM) can be applied to provide excellent estimates of original reservoir fluid compositions for conventional reservoirs. This methodology is very important in the cases at which an in-situ representative composition is not obtained. In this section, two practical ECM methods for unconventional reservoirs are proposed: One for initially undersaturated fluid systems (ECM1), and one for initially saturated fluid systems (ECM2). The ECM procedures can either be carried out by physical measurements or be computed with a properly tuned EOS model. These methodologies are compared to simply recombining separator samples at the initial producing GOR, a procedure commonly performed in the laboratory to predict in-situ fluid compositions.
Depleted, Initially Undersaturated Gas and Oil Reservoirs (ECM1)
If there is an initial production period in which the producing GOR (Rp) is constant, the in-situ fluid system is assumed to be undersaturated. When the fluid system is initially undersaturated, the solution GOR is known and equal to the initial producing GOR, Rsi=Rpi. The saturation pressure, however, is unknown. Hence, the solution GOR is used as a key matching parameter.
- First the produced wellstream is obtained by recombining the separator samples using the producing separator GOR.
- The recombined mixture is brought to equilibrium at the flowing bottomhole pressure (pwf) at the time of sampling, and reservoir temperature.
- All equilibrium gas is removed at constant pressure, and the equilibrium oil is stored.
- The equilibrium gas is mixed incrementally with the equilibrium oil, until the solution GOR is equal to the initial producing GOR, Rsi=Rpi. The resulting mixture is considered a very good estimate of the in-situ reservoir fluid composition.
Depleted, Initially Saturated Gas and Oil Reservoirs (ECM2)
If a GOR increase is observed immediately after the well is put online, the system is assumed to be initially saturated. When the fluid system is initially saturated, the initial saturation pressure is known and equal to the initial reservoir pressure, psat=pi. The initial solution GOR for both reservoir gas and reservoir oil, however, is unknown. Hence, the saturation pressure is used as a key matching parameter.
Saturation pressure of the produced wellstream is higher than the initial reservoir pressure:
- First the produced wellstream is obtained by recombining the separator samples using the producing separator GOR.
- The recombined mixture is brought to equilibrium at the initial reservoir pressure (pi=psat) and reservoir temperature.
- The resulting equilibrium oil is considered a very good estimate of the in-situ reservoir oil phase, while the resulting equilibrium gas is considered a very good estimate of the in-situ reservoir gas phase.
Saturation pressure of the produced wellstream is lower than the initial reservoir pressure:
- First the produced wellstream is obtained by recombining the separator samples using the producing separator GOR.
- The recombined mixture is brought to equilibrium at the current flowing bottomhole pressure (pwf) and reservoir temperature.
- All equilibrium oil is removed at constant pressure, and the equilibrium gas is stored.
- Equilibrium oil is incrementally mixed with the equilibrium gas until the saturation pressure reaches the initial reservoir pressure, psat=pi.
- The resulting gas composition (with dewpoint equal to the saturation pressure) is considered a very good estimate of the in-situ reservoir gas phase. The incipient oil phase/negative flash liquid (Whitson & Michelsen 1989) of this reservoir gas is considered a good very estimate of the in-situ reservoir oil phase.
This procedure was originally proposed by Reffstrup and Olsen (1994), which studied the problem of obtaining in-situ representative samples from low-permeability, saturated gas condensate reservoirs. Their methodology assumed a sample mixture with a dewpoint lower than the original dewpoint.