Coal Bed Methane - Ppt.pptx

OVERVIEW OF CBM & IT’S WORKOVER OPERATIONS

WHAT IS COAL BED METHANE ? •  Standard Natural Gas C • CBM is generated either through chemical reaction or bacterial action • Chemical reaction occur over time as heat and pressure are applied to coal in sedimentary basin, referred to as thermogenic methane. • Bacteria obtain nutrition from organic matter and produce methane as a by product, is referred to biogenic methane. • Methane is held in the cleats with the coal • Substantial water pressure is needed to keep the methane in the coal

FORMATION OF METHANE DURING COALIFICATION PROCESS

METHANE STORAGE IN COAL Methane in coal is: • Adsorbed on the surface of the coal • Stored as free gas in the cleats and open pores

Conventional Gas

Coal Bed Methane

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Inorganic reservoir rock

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Organic reservoir rock

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Reservoir and source rock independent

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Reservoir and source rock are same

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Gas storage in macropores, Real gas law

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Gas storage by adsorption on micropore surfaces

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Macropore Size

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Micropore size

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Darcy’s flow of gas to wellbore

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Diffusion through micropores by Fick’s law and Darcy’s flow through fractures

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Gas content from logs

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Gas content from cores and cannot obtain from logs

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Gas to water ratio decreases with time

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Gas to water ratio increases with time in stages

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Permeability not stress dependent

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Permeability highly stress dependent

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Well interference detrimental to production •

Well interference increases production

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Production Schedule according to set decline curves

Initial negative decline

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STAGES OF DEVELOPMENT (CBM)

1) Coring CORE ANALYSIS Gas volume Gas composition Coal description Adsorption isotherm Proximate analysis Ultimate analysis Vitrinite reflectance Bulk density determination

TECHNIQUES Conventional Coring, Wireline Coring, Pressure Coring.

2) Drilling & Well Logging 3) Casing

String Details Production Casing Hole size 7 7/8” or 8 ½” Casing size 5 ½” Setting depth +/- 1000m MD/TD Grade K-55 Weight 15.5 ppf Safety factor required Actual Load Design (based on strength) 1.1/1.0/1.34094.2/1460.5/929101.18/2.77/2.67 Design Assumption Burst Collapse Tension Frac Stimulation @3000 psi Full evacuation Running in hole @ 4ft/sec Connection BTC

1) Drilling 2) Casing & Cementation 3) Cased hole Logging & Perforation 4) Well Testing 5) Hydraulic Fracking 6) Well Completion 7) Production

Logging Analyses

 low density, coals are identified most easily from a density log  High apparent neutron and sonic porosities  High resistivities  Low gamma ray values

CBM PRODUCTION TECHNIQUES

METHANE DESORPTION • CBM is produced by removing water pressure that holds the CBM in place. • Methane that was held in place by water pressure.

PROGRESSIVE CAVITY PUMP • PCP is a positive displacement pump, where produced fluid is displaced axially at a constant rate. • PCP’S are comprised of two helicoidal gears, where the rotor is positioned inside the stator. • Combination of rotational movement and geometry of the rotor inside the stator results in the formation of cavities that move axially from pump suction to pump discharge.

Bottom Hole Equipment: • Stator • Rotor • No Turn Tool • Rotor bushing • Necessary X-overs & Gas anchors Surface Equipment: • Flow Tee • Hollow Shaft drive head • Intrinsically safe electric motor/hydraulic motor driven by duel fuel engines • Polished rod • Polished rod clamp

PRODUCTION PROBLEMS & NEED FOR WORKOVER • Workover refers to any kind of well intervention involving invasive techniques

Parameter  

Causes Low water

 

head in the

 

Annulus Sand

High Torque

• Most common workover operations includes Sand cleaning & Flushing, Downsizing the pump, Fishing, etc..,

Sand Cleaning

Production

• It can also be described as the process of performing major maintenance or remedial treatment on a oil or gas well. • In all the operations, pump is required to stop and in some of the operations downhole pump is to be retrieved.

Workover type Downsize The Pump

      Low Torque (Mostly Means Fishing)

Low Production

Increases Gas Influx In

Lower the PID

Tubing Elastomer

Downsizing Rotor

Swelling Pump

(Slim Hole Rotor) Fishing

Unscrewed Sucker Rod

Fishing

Unscrew   Tubing / Sucker

Fishing

Rod Shear NTT Fail

Change NTT –

Tubing

Fishing Replace Tubing

Puncture

CONSTRAINTS OF CBM AND THEIR POSSIBLE SOLUTONS • Failure in Attainment of Continuous Dewatering (Due to sand production, scaling, sucker rod unscrew, tubing puncture, high torque etc..,) SOLUTION: Eliminating workover is an impossible target but it can definitely be reduced. I.

if a high percentage of sand is pumped out continuously, it may lead to abrasive wear of the rotor and stator. So running a pump with larger displacement per RPM and lower pump speed reduces velocity of the particles within the pump preventing abrasive wear.

II.

The well must be properly swabbed after acid job till the pH of the fluid is back to its original level to avoid acid attack on the rotor and stator.

III. Proper space-out is mandatory keeping in mind every small factor to avoid the pump locking up and other torsional stresses. IV.  To avoid elastomer swell, dry run and burning of the elastomer due to friction, the selection of pump model, elastomer-fluid compatibility check and PID selection must be appropriate so that it does not require frequent changes. V.

If dry run is the result of high percentages of free gas, lowering the pump setting depth or running a gas anchor will limit the gas entry.

VI. There are some mechanical failures like tubing punctures and sucker rods being worn out, and to reduce this, centralizers should be used for each sucker rod and the pump should be operated at an optimum RPM which will decrease the vibrations.

CONTINUED…

•LOW PERMEABILITY- (permeability of CBM is extremely low i.e. below 0.1 mD) SOLUTION- H F • SAND PRODUCTION SOLUTION- If the permeability even after the HF job is low, therefore gravel packing cannot be used. The only changes which we can make is increasing the total depth of the well i.e. the sump. •SCALE FORMATION SOLUTION: Injection of biodegradable scale inhibitors in the formation to reduce frequent acid jobs. •WATER INFLUX DURING DRILLING SOLUTION: Shift from air drilling to mud drilling. •SURFACE WATER DISCHARGE SOLUTION: Before disposal it has to be treated, as it contains Trace metals and organics which are carcinogenic .

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