Elements of gas well monitoring are described as:Generating accurate reserves estimates Production forecasting, Well testing, Work over planningGas wells are usually cauterized by:Low pressure draw downs and high well head pressuresThis needs extreme care in surface handling of gas from technical as well as economic considerations.The situations are encountered in the gas wells may be given as:

1. Consistent production: if the formation has good permeability.

2. Intermittent production: If the formation is very tight* and thus the permeability is very low.

Conventionally,1. Well efficiency defined as absolute open flow potential and

deliverability of well**2. The monitoring of gas well is given by rate of production.

* Tight gas reservoirs are generally defined as having less than 0.1 mille darcy ( m D) matrix permeability and less than ten percent matrix porosity.** Deliverability of a gas well is generally taken as 25% of AOFP. The deliverability of a given facility is variable, and depends on factors such as the amount of gas in the reservoir at any particular time, the pressure within the reservoir, compression capability available to the reservoir, the configuration and capabilities of surface facilities associated with the reservoir, and other factors. In general, a facility's deliverability rate varies directly with the total amount of gas in the reservoir: it is at its highest when the reservoir is most full and declines as working gas is withdrawn. Deliverability- is most often expressed as millions of cubic feet per day (MMcf/day). It is also expressed in terms of equivalent heat content of the gas withdrawn from the facility, most often in dekatherms per day (a therm is 100,000 Btu, which is roughly equivalent to 100 cubic feet of natural gas; a dekatherm is the equivalent of about one thousand cubic feet (Mcf)).

During the primary production period of a gas field the operator usually is not aware of any gas loss from the reservoir, such as loss to shallow or deeper formations by means of well bore communication gas, or vented from surface production equipment, or gas lost by leaks in such equipment. This may be carried out by periodical rate/pressure determination of the gas from the specified well.The measurement/monitoring must ensure:- Sustained gas production- Preferably no water production- No sand production, and- Least vibrations to well head equipment.The basic Darcy law for a radial gas flow is given as:

------------ (1)Whereqgr = gas flow rate at radius r, bbl/day r = radial distance, fth = zone thickness, ft µg = gas viscosity, cp p = pressure, psi0.001127 = conversion constant from Darcy units to field units

As the gas flow rate is usually expressed in scf/day. Referring to the gas flow rate at standard condition as Qg, the gas flow rate qgr

under pressure and temperature can be converted to that of standard condition by applying the real gas equation-of-state to both conditions, or

or ------------- (2)Wherepsc = standard pressure, psia Tsc = standard temperature, °RQg = gas flow rate, scf/day qgr = gas flow rate at radius r, bbl/dayp = pressure at radius r, psia T = reservoir temperature, °Rz = gas compressibility factor at p and Tzsc = gas compressibility factor at standard condition ≅ 1.0

Combining Equations 1and 2, we get:

Considering Tsc = 520 °R and psc = 14.7 psia, the equation yield:

------------------------- (3)Integrating Equation (3) from the wellbore conditions (rw and pwf) to any point in the reservoir (r and p) to give:

------------------------- (4)Imposing the condition of Steady-state flow which requires that Qg is constant at all radii and Homogeneous formation which implies that k and h are also constant, the above equation is transformed as:

where

Therefore,

--------------- (5) Integral oʃ p ( 2p / µg z) dp is called the real gas potential or real gas Pseudo pressure and usually represented by m(p) or ψ. i. e.

------------ (6) and the equation (5) gives:

or ----------- (7)

The equation 7 is a straight line between ψ vs. ln r/rw form (5) where slope is given by Q g T/0.703kh and ψ as intercept. Thus Qg may be given as:

------------------------- (8) in the particular case when r = re , then :

----------------------------------------------- (9)

Where ψe = real gas potential as evaluated from 0 to pe, psi 2/cpψw = real gas potential as evaluated from 0 to Pwf, psi2/cpk = permeability, md h = thickness, ft.re = drainage radius, ft. rw = wellbore radius, ft.Qg = gas flow rate, scf/day

Further, when gas flow rate is expressed in Mscf/day, we get

-------------------------------------------- (10)

Where Qg = gas flow rate, Mscf/day, In terms of average pressure pr ,the equation changes as:

--------------------------- (11)Modification:The exact gas flow rate , which incorporates the term 2/µz may be modified as constant(?), the equation (11) may be written as:

Removing the term 2/µz and integrating, we get :

------------------------- (12)WhereQg = gas flow rate, Mscf/day and k = permeability, mdThe term (µg z ) avg is evaluated at an average pressure p, given as

Note:This modified method is called the pressure-squared method.*** Only for the pressure range of < 2000 psiExample

The following is the PVT data from a gas well :

The well is producing at a stabilized bottom-hole flowing pressure of 3600 psi. The wellbore radius is 0.3 ft. The following additional data is available:k = 65 md h = 15 ft. T = 600°R pe = 4400 psi re = 1000 ft.Calculate the gas flow rate in Mscf/day.SOLUTION:TWO OPTIONS EXISTSOPTION-1 Exact Calculations

Step 1. Calculate the term[ 2p/µgz ] for each pressure as below:

Step 2. Plot the term[ 2p/µgz ]versus pressure as shown in Figure:Real gas pseudo pressure data

Step 3. Calculate numerically the area under the curve for each value of p. These areas correspond to the real gas potential ψ at each pressure. These ψ values are tabulated below and ψ versus p is also plotted in the figure above.

Step 4. Calculate the flow rate by applying Equation-8

OPTION-2 Approx. CalculationsThe exact gas flow rate as expressed by the above method can be approximated by removing the term [2/µgz] outside the integral as a constant. Step 1. Calculate the arithmetic average pressure.

Step 2. Determine gas viscosity and gas compressibility factor at 4020 psi.

Step 3. Apply Equation

Remarks:Results show that the pressure-squared method approximates theexact solution of 37,614 with an absolute error of 1.86%. Thiserror is due to the limited applicability of the pressure-squaredmethod to a pressure range of <2000 psi.