NEW MODEL ADDS PRECISION TO GAS-LIFT DESIGN

R.A. Kendrick
Hampton Resources Inc.
Houston

A.H. Woodyard, J.W. Hall
Conoco Inc.
Houston

Conoco Inc.'s new analytical technique for lift gas allocation identifies, in one-pass, injection rates and the achievable mandrel location for injection.

The injection depth is first fixed at each mandrel location and a nodal analysis is used to establish a range of possible injection and production rates. The mandrel-specific response curves are combined to create the well's total response curve.

Current gas-lift allocation techniques do not determine production rates for discrete mandrel locations. Allocation rates for particular wells are made on the basis of a fixed differential pressure. When actual mandrel locations are superimposed on these solutions, gas often must be reallocated.

The advantages of the new technique include:

Determining the transfer capability of the gas-lift valve in each mandrel
Finding valve pressure drop as a function of injection gas rate
Obtaining a more realistic response curve.

Another potential benefit is that the response curve can be adjusted to reflect the water cut and/or multizone completion effects at different injection depths. Because the node is at the mandrel, the inflow performance relationship (IPR) at that depth can easily be adjusted to include such effects.

GAS-LIFT APPLICATIONS

Proper application of gas lift is an important activity for Conoco which operates in more than 25 countries on six continents.

Worldwide, Conoco artificially lifts almost 500,000 bo/d. Gas lift contributes about 80% of this production from just 10% of the nonflowing wells.

Because a few wells produce most of the artificial lift production, gas lift is a key component of production systems engineering, a core technology at Conoco.

To exploit gas lift, Conoco has formed a gas-lift team committed to communication, involvement, and working across organizational boundaries.

This article covers a technique that the team developed for improving gas-lift well analysis, mandrel spacing techniques, and valve installation design. The technique is incorporated in a computer model that reflects actual, rather than idealistic, production response to liftgas injection.

As this technique is transferred to the operating personnel, it will enable Conoco to improve gas-lift efficiencies and ultimate recovery of hydrocarbons.

RESPONSE CURVES

Traditional gas-lift allocations have been based on nodal analysis techniques. For a range of injection rates, nodal analysis seeks the intersection of IPR curves with tubing performance curves.2 The intersection represents the theoretical production that can be expected from any gas-injection rate (e.g., a response curve).

Because the response curve has been noted to be affected by injection depths a more rigorous treatment is required on how gas-lift mandrel locations determine the valid range of production and injection rates for the response curve.

The new technique allows the nodal analysis "node" to be set at each mandrel depth in turn, rather than fixing the node at the completion depth. This establishes a range of production and injection rates at each gas-lift mandrel depth.

The result is a mini-response curve unique to each mandrel location. Overlaying the mini-response curve for each mandrel creates the well's response curve.

What are the characteristics of a response curve?

In gas-lift allocation the curve represents the fluid production the well provides in response to injected gas volume. Fig. 1 illustrates the classic shape that is characterized by three areas: transfer, optimum, and inefficient.

The transfer section depicts how a well typically responds to initial gas injection rates. A positive steep slope suggests that a small increase in injection rate will provide a disproportionate increase in production.

The optimum-injection rate section (zero slope) is determined through proper analysis and operation. The rate is often reached at the expense of other wells. This section is especially critical when there is a concern about system back pressure and limited lift-gas supply.

In the last section, overinjection can cause inefficiencies that decrease production as more injection gas is supplied (negative slope).

How to create this curve is well known.

A nodal analysis that matches tubing performance against IPR performance over a range of injection rates is plotted as Fig. 2. Each intersection point on that figure is characterized by two values: a production rate and an injection rate that is found by multiplying the injected gas to produced liquid ratio (IGLR) for the tubing performance curve by the production rate at the intersection.

The two values, production rate-vs.-injection rate, create the response curve.

INJECTION DEPTH

The response curve of Fig. 1 uses a required differential pressure between the injection and production streams.

Reference 4 points out that the response curve must be adjusted for changing well parameters including injection depth. However, traditional techniques for optimizing well performance use an injection depth where the differential pressure is satisfied.

This injection depth does not have to lie at a preset mandrel depth. Fig. 3 illustrates this with a differential pressure that corresponds to an operating point at some depth other than a mandrel depth.

When the desired differential pressure is satisfied, the well may be multipointing. But if the well is unable to maintain that pressure differential, the gas may not reach the deepest mandrel, Mandrel 6 in Fig. 3.

Fig. 4 shows the response curve with the mandrel locations corresponding to the injection rates found from the nodal analysis in Fig. 2. Note that as the injection rate increases, the injection depth is changing. If the injection depth is plotted against injection rate (Fig. 5), the injection rates do not correspond to a mandrel depth.

Fig. 5 is drawn as a continuous line but, in reality it must be a discontinuous line. As the injection rate changes the injection point will be transferred to the next mandrel.

Depending on valve design, this happens when one valve closes as another opens or when the well is multipointing.

No matter how the transfer is accomplished, Fig. 5 illustrates the shortcoming that only the lifting depth and not the lifting mandrel can be found with current allocation techniques.

FIELD-WIDE CURVES

Lift gas on a field-wide basis is allocated in two steps.

During allocation of a limited amount of lift gas, the injection rates are first selected so that the slope of each well's response curve is identical. The sum of the injection rates selected must equal the limited lift gas available.

Note that the allocation points are selected by slope and rate without regard to injection depth.

Often, when returning to the individual well with the allocated gas-injection rate, it is found that the required injection depth does not correspond to a mandrel depth (Fig. 5).

To overcome this inconsistency, a second step must adjust the injection depth to a known mandrel depth (usually the next shallower location). After this adjustment, the corresponding injection rate is calculated.

Repeating this process for every well yields the feasible mix of rates and injection depth.

Unfortunately, because the allocated rates have been adjusted to reflect feasible injection depths (i.e., mandrel depths), a reallocation must be performed. The reallocation uses a new set of response curves created with the injection depth limited to the feasible mandrel location.

This second pass ensures that the gas allocated to each well can be supported by a valid mandrel depth. It may however, change the differential pressure for each well's injection point.

MANDREL CURVE

The preceding reallocation process can be eliminated if the response curve for each well is initially created for a fixed depth of injection. Fig. 6 illustrates this approach.

The computer program which calculates the response curve fixes the injection depth and allows the differential pressure between tubing and casing to change.

The differential-pressure range must accommodate the valid range of production and injection rates for that injection depth.

With the injection depth fixed, the response curve ends when the injection rate signals a transfer to the next mandrel. This is repeated for every mandrel that is within the valid operating range.

Field-wide allocation using mandrel response curves quickly identifies the injection depth when entering the curve with a desired allocation slope (barrels produced per cubic foot of gas injected). The response curve for the deepest mandrel that satisfies the allocation slope will be chosen.

A response curve for every mandrel in the well is unnecessary. With a given static bottom hole pressure and completion drawdown, the shallowest mandrel available to sustain flow can be estimated. For wells without large drawdowns, this check reduces, and occasionally eliminates, almost all the mandrels available for response-curve generation.

Similarly, the analysis may also eliminate mandrels located too deeply in the hole to be reached with the available rates.

VALVE DESIGN

After the well response for each mandrel depth is fixed, a valve must be in place that can act on the pressures and volumes established from the analysis. Conoco's gas-lift team has given considerable attention to proper selection of valve operating characteristics.

Fig. 7 contrasts the two operational extremes for gas-lift valves. The "transfer" curve identifies the idealized valve performance that is predicted in the transfer model discussed here. A constant casing pressure is required.

The "throttling" curve is a more typical valve performance.

When the node is fixed at a mandrel, the resulting nodal analysis will record the injection rate, casing pressure, and tubing pressure that satisfy the chosen production rate.

As the production rate changes, both injection-gas flow rate and the tubing (downstream) pressure for the mandrel depth are recorded. The results are plotted with the axes of Fig. 7 and assist in finding the valid operating characteristics of the valve to be chosen for that mandrel depth.

ACKNOWLEDGMENTS

The authors thank Conoco for its support of the gas-lift team.

REFERENCES

Prahalad, C.K., and Hamel, G., "The Core Competence of the Corporation," Harvard Business Review, May-June 1990, Number 3.
Simmons, W.E. "Optimizing Continuous Flow Gas Lift Wells," Petroleum Engineer, August 1972.
Mach, Joe, et a]., "A New Concept in Continuous Flow Gas Lift Design," JPT, December 1983.
Ferrer, A.A., et al., "Use of a Computerized Model in the Optimization of a Continuous Gas Lift Operation," Paper No. SPE 21641, 1991.
Woodyard, Alan, "Gas lift workstation improves productivity," OGJ, Sept. 4, 1989.