FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
THE PATENTS RULES, 2003
Provisional/ Complete specification
[See section 10 and rule 13]
1. Title of invention:
"Venturi Type Surface Chokes for Stabilized flow"
2. Applicant(s):
Name Nationality Address
Oil and Natural Gas India IOGPT, Phase - II, Panvel -
Corporation Ltd. 410221, Navi Mumbai,
Maharashtra, India.
3. Preamble to the description:
The following specification particularly describes the invention and the manner in which it is to be performed.

VENTURI TYPE SURFACE CHOKES FOR STABILIZED
FLOW
BACKGROUND OF THE INVENTION
1. FIELD OF INVENTION:
The present invention relates to a surface flow choke for obtaining better flow efficiencies in oil / gas wells.
2. DESCRIPTION OF PRIOR ART:
Presently chokes having same internal diameter from entry to exit point ("square edge surface chokes") (FIG. 1) are being used for flow control in oil /pas wells. When the sguare edge surface chokes are used, the critical flow is achieved when the well fluid pressure just after the choke i.e., towards flow line / separator facility ("downstream pressure") of the choke is approximately less than 50% of the well fluid pressure before the choke i.e., pressure towards the well ("upstream pressure"). When the flow is critical, the flow rate is stable, as the changes In the downstream pressure of the choke do not affect the pressure upstream of the choke. In actual field operating conditions if the downstream pressure is higher than 50% of upstream pressure, then the flow wduld be sub-critical. In sub-critical flow, the flow rate is affected by the fluctuations in pressures downstream of the choke. Due to this the upstream pressure of the choke also gets affected. The increase in the downstream pressure increases the upstream pressure. Any increase in the upstream pressure will decrease the gas flow rate from the well and in some ca,ses result in ceasure of the gas wells due to loading on account of reduced gas flow rates.
Decline in reservoir pressures over the production life of a well and necessity of a higher separator pressure to meet the operational

requirements would make the well flow to enter into the undesired sub-critical region.
3. DETAILED DESCRIPTION OF THE INVENTION:
The gas flow rate through a choke is calculated by the THORNHILL CRAVER equation, where the flow rate primarily depends upon the cross sectional area of choke, upstream pressure and ratio of downstream to upstream pressure. The equation is as follows;

Where
r - Pdn / P up
Q - Gas flow rate in MSCFD
A p - Cross section area of choke in square inches
Pup - Upstream pressure in psia
Pdn - Downstream pressure in psia
Tv - Temperature of gas, Deg R
K - Specific heat ratio
Sg - Specific gravity of gas.
Cd - Coefficient of discharge
The "Cd" value accounts for the choke geometry and multidimensional flow effects.
The present invention relates to a choke having convergent profile at the entry of the choke and a throat portion of uniform diameter followed by a divergent profile ("venturi profile") (FIGS. 2 and 3) for achieving maximum flow rate through a choke ("critical flow") with less pressure differential between upstream and downstream pressures across the choke. The choke has a convergent divergent type of profile for which continuity equation along with equation of state can be applied.

As per the continuity equation, applied to throat section of a convergent divergent profile, the maximum flow rate will be, m = At* Dt *Vt, i.e. product of area, density and velocity at throat section.
At throat section, the velocity will be velocity of sound and mach number will be 1. As per the continuity equation, the mass flow rate remains the same at throat and exit, and only the velocity and area vary accordingly.
The change in velocity will lead to the change in pressure, which means that a pressure recovery can be induced at the exit by varying the choke profile.
From the throat diameter of the modified profile (equal to normal choke size), the choke diameter is increased with a divergent section so that there will be a pressure recovery at the outlet. The pressure recovery will allow the choke to reach critical flow with a comparatively low pressure differential between upstream and downstream pressure. At the throat section, the mach number will be equal to 1 and the flow rate will be maximum. Once critical flow is achieved at the throat, the flow rate will not change for further reduction in downstream pressures.
The basic equation for mass flow rate through orifice will be M = FPc At/(K R T) 1/2, where K = Cp/Cv Where F- flow function
= 2 K/K-1 (Pt/Pc) 2/K (1 - (Pt/Pc) K-1/K) 1/2
Considering a divergent section and the throat of a modified profile, as per continuity equation,
M = At(K Dt Pt) 1/2 (2 / (K+1)) K+1/2 (K-1) (flow at throat) = Ae (2K/(K-1) D Pt) 1/2 ((Pe/Pt) 2/K - (Pe/Pt) K+1/K) 1A

(flow at exit)
Equating the two and using isentropic relations the area ratio of exit to throat and pressure ratio in terms of K and mach number is obtained.
A/At = 1/M ((2/(k+1)) * (1+ ((K-1)/2) * M2)) k+1/2(k-1)
P/Pup = (1+ ((k-1)/2)*M2) k/k-1
The area ratios were fixed based on this equation and angle of diverging section was calculated based on exit diameter and the length of divergent section.
The dynamic flow performance of venturi type chokes have been evaluated with varying throat lengths vis-a-vis throat diameters and varying convergent section based on De-Laval Nozzle or convergent-divergent nozzle, (a tube that is pinched in the middle, making an hourglass-shape). The experiments were conducted with Nitrogen gas with pressures up to 50 Kilogram per square centimeter.
• The experiment results showed that the critical flow is achieved even at a higher downstream pressure of 90% of upstream pressure across the venturi profile choke and the critical flow rates are approximately 15% higher than the theoretical maximum flow rate through the standard square edge choke of same choke diameter/throat dimension. (FIG. 4)
• Two types of flow efficient venturi choke profiles are developed for choke sizes 3/16", Y/ and 5/16". (Table 1 & 2 below)

Table-1 (Profile 1) (FIG. 5)

Size Critical Rate achieved at Pdn / Pup ratio Critical Gas Flow ratio ( Actual / Theoretical) %
3/16" 89.30 114.5
1/4" 91.87 117
5/16" 89.43 114
Table-2 (Profile 2) (FIG. 6)

Size Critical Rate achieved at Pdn / Pup ratio Critical Gas Flow ratio ( Actual / Theoretical) %
3/16" 91.5 115.58
1/4" 89.23 113.83
5/16" 88.46 114.73
In the square edge surface chokes, the critical flow is achieved when the downstream pressure of the choke is approximately 50% of the ("upstream pressure") whereas the surface chokes with venturi type profile achieves the critical flow even at a higher downstream pressure of 90% of the upstream pressure.
4. FIELD APPLICABILITY:
• A Venturi profile will allow the chokes to be operated in the critical region and yet will pass the maximum flow rate up to downstream pressure of 90% of upstream pressure across the choke. Any pressure fluctuations in downstream between 50 - .90% of upstream pressure will not affect the gas throughput and thereby eliminate instability in the upstream pressure. This results in stabilizing the gas production and thereby minimizes the ceasing of the gas wells due to water loading.
• Allows passing up to 15 % more gas than conventional square edge chokes of same choke diameter / throat dimension.
• Can be used in oil wells also for stable flow. Allows wells to be operated at higher separator pressure to meet operational requirements without loss in the production due to increase in back

pressure. This would in turn also help in cost reduction on account of Low Pressure (LP) gas compression, as the gas would be available at a higher pressure for suction end.
5. SUMMARY OF INVENTION:
A choke with a venturi profile achieves critical flow rates at a down stream pressure of 90% of upstream pressure across the choke. The most important benefits envisaged by the invention are --
• Gas wells: Can be operated at higher separator pressures without loss in gas production and thereby prolonging the life of gas wells by minimizing the possible liquid loading on account of reduced gas flow rates at higher back pressure.
• Oil wells: Can be operated at higher separator pressures without loss in oil production and subsequent cost reduction on account of LP gas compression.

BRIEF DESCRIPTION OF DRAWINGS:
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
1. FIG. 1 is a schematic illustration of the dynamic gas flow performance of the Square Edge Surface Choke.
2. FIG. 2 is a schematic illustration of the dimensions of Venturi Profile Choke.
3. FIG. 3 is a schematic illustration of the dynamic gas flow performance of the Venturi Surface Choke.
4. FIG. 4 is a diagram representing the Comparison of Dynamic Gas Flow Performance through Square Edge and Venturi Type Chokes
5. FIG. 5 is an illustration of the dimensions of Venturi Profile no.1.
6. FIG. 6 is an illustration of the dimensions of Venturi Profile no.2.

WE CLAIM
1. A surface choke with a venturi profile as provided below achieves critical flow rates at down stream pressure of 90% of upstream pressure across the choke obtaining better flow efficiencies in oil / gas wells.

2. A venturi profile surface choke with the dimensions as set out below achieves critical flow rates even at down stream pressure of 90% of upstream pressure across the choke obtaining better flow efficiencies in oil / gas wells.

Choke Size A
mm B mm C mm D mm E mm F mm a Deg Deg
3/16" 7.5 2.5 15 11.9 4.76 7.4 25.4 5.03
1/4" 10 3.2 22.64 16 6.35 10.35 25.4 5.048
5/16" 12.4 4 15 20 8 11.2 25.4 6

3. A venturi profile surface choke with the dimensions as set below achieves critical flow rates even at down stream pressure of 90% of upstream pressure across the choke obtaining better flow efficiencies in oil / gas wells.

Choke Size A mm B mm C mm D mm E mm F mm a Deg P Deg
3/16" 7.5 2.5 15 23 4.76 7.4 50 5.03
1/4" 10 3.2 22.64 30 6.35 10.35 50 5.048
5/16" 12.4 4 15 38 8 11.2 50 6
4. The critical flow rate through a venturi profile choke is approximately 15% higher than the theoretical maximum flow rates through the standard square edge chokes of same choke diameter/ throat dimension.