MOMENTUM KILL PROCEDURE CAN QUICKLY CONTROL BLOWOUTS

W. David Watson
Southern International Inc.
Oklahoma City

Preston Moore
Preston L. Moore & Associates Inc.
Norman, Okla.

The momentum kill method can help in quickly regaining control of a blowing well, providing the blowing well rate and fluid properties can be estimated reasonably.

A momentum kill is the practical application of fluid dynamics to the task of killing an uncontrolled blowing well.

The method was introduced to the industry in 1977 and has since been successfully applied to a number of wild wells, many of which would have otherwise required a kill with a relief well.1 This method is an attractive technique for controlling a well because it can save money, time, effort, and natural resources compared to other blowout control methods.

The momentum of the kill fluid counteracts and overcomes the flowing momentum of formation fluids. In other words, sufficient mud density pumped at a sufficient rate is directed into the flow stream to force the escaping fluid column back into the well bore. Sufficient kill fluid hydrostatic pressure must be "stacked" in the hole so that the well remains dead after the operation.

The momentum kill is not a panacea for all blowouts. An assessment must be made of the potential problems unique to this method, and certain requirements must be met if the technique is to be successful.

The following are some of the considerations for evaluating the use of the momentum kill method:

• Some means of introducing kill fluid directly into the flow conduit must be available. The ability to run a kill string to some relatively shallow depth can greatly diminish the kill fluid density and pump rate requirements in the case of a gas blowout.

• High pump rates and high mud densities are usually required. These factors necessitate high pump pressures; thus, some consideration must be given to the burst ratings of the well bore tubulars.

• A reasonable estimate of the production rate from the well is necessary.

• The nature and physical properties of the produced fluids must be known to some extent.

• The pore pressure of the producing formation must be known so that sufficient hydrostatic pressure can be achieved after the kill has been effected.

THEORETICAL BASIS

The technique derives from the impulse-momentum principle from basic fluid dynamics. The principle is one way of expressing Newton's second law of motion (Equation 1). In other words, the force exerted on a fluid system is equal to the rate of change in the fluid's momentum.. Assuming steady state (constant mass flow), this equation can be written as Equation 2.

Consider a system wherein a liquid is pumped into a vertical well and the liquid impinges on an opposing flow of formation fluid. The desired result of a momentum kill is a state of equilibrium at the point of impact. That is, the fluid velocity in each opposing system reduces to zero, and the resultant vertical forces for the two fluids are equal but act in opposite directions. Equation 2 may be rewritten as Equation 3, with v representing the change in velocity because DELTAv = v - 0.

The term m/DELTAt is the mass rate of the fluid and can be expressed in terms of the fluid density and the volumetric flow rate (Equation 4). The fluid velocity is a function of the cross-sectional area of the flow conduit (Equation 5). Equation 6 is the area of the flow conduit, assuming it is a tube of inner diameter Dc.

Substituting and applying conversion constants leads to the momentum force equation of the kill fluid (Equation 7). Equation 7 also applies to the flow of such relatively incompressible formation fluids as salt water. In Equation 7, Fmo is the momentum force in lb, p is liquid density in ppg, Q is the liquid flow rate in bbl/min, gc is 32.17 lb-ft/lb-sec2, and Dc is the flow conduit diameter in inches.

An application of this relationship for a hypothetical saltwater blowout is presented in the accompanying box.

GAS BLOWOUT

In a gas blowout, the formation fluid compressibility must be considered because the compressibility of the gas introduces variability to the density and velocity terms. The gas momentum then depends on the gas pressure and, hence, on the location of the gas in the well bore. Consider the gas momentum at surface and the simplified case of gas exiting a well at standard pressure and temperature conditions. The gas density is given by Equation 8.

The molecular weight of the gas is equal to the product of its specific gravity and the molecular weight of air (Ma), and the gas constant (R) depends on the selected unit system. Ma = 28.97 lb/lb-mole and, for common oil field units, R = 10.73 psia-cu ft/lb-mole-R. Using Psc = 14.65 psia, Tsc = 520 R, and Zsc = 1.0, the density of a gas at standard conditions is given by Equation 9. Note that the psc units in this equation are lb/cu ft.

Substituting into Equation 3 and applying conversion constants gives the momentum force of gas at standard conditions (Equation 10). Qsc is the gas flow rate (MMcfd) at standard pressure and temperature.

It is assumed standard gas conditions will be inadequate in most cases because high flow rates typically involve high surface flowing temperatures. Also, it may be necessary or desirable to effect the kill at some depth in the well other than near surface.

The gas momentum is a function of the gas density and velocity which, in turn, depend on the prevailing pressure and temperature. These terms must therefore be determined at actual conditions.

If the temperature of the surface stream is known, the flowing temperature at any depth can be estimated with reasonable accuracy by assuming a constant gradient to the formation temperature. Some of the common published rules of thumb or analogy in a given area may also prove to be useful. In any event, accurate temperature estimates are not essential to obtaining valid results.

More important to the predictions is having some means of determining the pressure at a particular depth. A key parameter to the solution is the flow rate (Qsc), and a reasonable estimate of this quantity is essential.

The volumetric flow rate at any pressure and temperature can be calculated with Equation 11, which is based on the conservation of mass and the gas law. The gas density at P and T is now calculated by Equation 12, with pg in lb/cu ft. Substituting Q and pg into Equation 3 and converting units gives the momentum force equation for a gas at any pressure and temperature (Equation 13).

Several correlations have been published for prediction of flowing pressures and gas compressibility factors in a well bore, and most require extensive iteration to arrive at a solution. The well bore pressures during a blowout are normally relatively low, which justifies some simplification of the established methods. Under low pressure conditions, an average temperature and Z factor can be assumed for the gas column with little sacrifice in accuracy. The Cullender and Smith relationship for flowing pressures may then be simplified (Equation 14).2 In this equation, P, is the surface flowing pressure in psia, L is the length of the flow conduit to the depth of interest in ft, H is the true vertical depth of interest in ft, and f is the Fanning friction factor.

Substituting Weymouth's friction factor correlation (Equation 15) for f in Equation 14 yields Equation 16.

Extremely high gas rates may exceed the practical limitations of a kill fluid density and pump rate. In these situations, the required momentum force of the kill fluid can be substantially reduced if the flow impacts the gas stream at some depth in the well. For a vertical well (H = L), Equation 16 may be rearranged to solve directly for the kill string depth (Equation 17). (The terms a, b, and c are used for convenience and are defined in the nomenclature).

The minimum depth to place the kill string under a given kill fluid momentum can be obtained by iteration of Equations 14 and 17. First, the flowing gas pressure required to balance the available momentum of the mud can be estimated using Equation 14. The depth at which this pressure occurs can then be solved using Equation 17. This procedure is illustrated in the gas blowout example.

MOMENTUM KILL PROCEDURE

In practice, killing a well with this method is an optimization process. A range of mud densities and pump rates are selected, and the kill string depth requirement is calculated for each set of mud parameters. Ultimately, the optimum mud density, pump rate, and kill string depth can be determined by considering the pumping pressure limitations, mechanical condition of the, well, job logistics, and other factors.

The following procedure is recommended for a momentum kill:

• Determine momentum profile of the well flow.

• For various candidate kill fluids and kill strings, calculate and plot the depth of injection vs. the required kill rate and pumping friction pressure.

• Determine the optimum kill string, kill fluid, injection depth, and pump rate.

• Rig up and install the kill string at the selected depth.

• Prepare the kill fluid, and commence injection at the kill rate.

• When the well flow ceases, vent all strings and observe.

• Re-enter and repair the well.

EXAMPLE

The following is an example of a successful momentum kill. A well is freely flowing a stream of gas, condensate, water, and particulate matter up 7 5/8-in. casing at high velocity. The flow is escaping to the atmosphere, and the flow rate is unknown. Table 1 lists the gas properties.

The mechanical condition of the well precludes running a kill string into the well. A stinger will be required for this operation because a kill must be effected near the surface. The intent is to restrict the annular flow to the point where a minimal amount of kill fluid is lost during the pump job. An effective stinger may consist of only a few joints of pipe.

Assuming the density and velocity of the flowing fluid stream, the momentum force can then be calculated, and the counter momentum force of the kill fluid can then be determined. Figs. 1 and 2 present the required pump rates and kill fluid densities for a range of gas flow rates for this well.

REFERENCES

1. Grace, R.D., "Practical Considerations in Pressure Control Procedures in Field Drilling Operations," Journal of Petroleum Technology, August 1977, P. 1,031.

2. Cullender, M.H., and Smith, R.V., "Practical Solution of Gas-Flow Equations for Wells and Pipelines with Large Temperature Gradients," Transactions of AIME, Vol. 207, 1956, p. 281.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.