Lesson 12 Well Engineering.

Lesson 12 Well Engineering

Harold Vance Department of Petroleum Engineering
Well Engineering Circulation Programs Circulation Calculations (air, gas, mist) Circulation Calculations (gasified liquids) Harold Vance Department of Petroleum Engineering

Well Engineering Wellhead Design Casing Design Completion Design Harold Vance Department of Petroleum Engineering

Well Engineering Bit selection Underbalanced perforating Drillstring design Separator design Harold Vance Department of Petroleum Engineering

Hole Cleaning Optimizing hydraulics with gasses is primarily concerned with hole cleaning - getting the cuttings that are generated by the bit out of the hole. With gas, rheological properties have very little to do with hole cleaning Hole cleaning with gasses is almost entirely dependent on the annular velocity Harold Vance Department of Petroleum Engineering

Drag and Gravitational Forces
Flowing air exerts a drag force on cuttings Gravitational force on the cuttings Therefore there is a threshold velocity in which the cuttings will be lifted from the wellbore. Threshold velocity increases with size of cuttings. Harold Vance Department of Petroleum Engineering

Hole cleaning Compressibility of air (or gas) complicates matters. Frictional pressure increases downhole pressure - decreases velocity downhole Suspended cuttings increase the density of the air, increasing downhole pressure. Temperature has an effect on volumetric flow rate. Harold Vance Department of Petroleum Engineering

Hole Cleaning We must pump at a velocity high enough to remove the cuttings, but not too high where we waste energy. Harold Vance Department of Petroleum Engineering

Hole Cleaning Criteria
Terminal Velocity Criteria Minimum Energy Criteria Minimum BHP Criteria Harold Vance Department of Petroleum Engineering

Terminal Velocity Criteria
Gray determined that the minimum velocity of the gas must be at least as high as the terminal velocity of the cutting in order to lift the cutting from the wellbore. Vc = Vf - Vt Harold Vance Department of Petroleum Engineering

Terminal velocity Harold Vance Department of Petroleum Engineering

Terminal Velocity Harold Vance Department of Petroleum Engineering

Terminal Velocity Terminal velocity in air drilling is determined mainly by: cutting diameter, shape, and density bottom hole temperature and pressure Harold Vance Department of Petroleum Engineering

Terminal Velocity As pressure increases Vt decreases. As pressure increases Air velocity decreases If the mass flow rate of gas remains constant the local air velocity decreases with increasing pressure. The air flow rate required to lift the cuttings increases with increasing BHP Harold Vance Department of Petroleum Engineering

Friction Pressure Eq. 2.5 Harold Vance Department of Petroleum Engineering

Friction Pressure Harold Vance Department of Petroleum Engineering

Friction Pressure Mixture density is a function of air density, cuttings density, and mass of the cuttings. Air density is a function of the pressure Mass of the cuttings is a function of: ROP Hole cleaning efficiency Harold Vance Department of Petroleum Engineering

Friction Pressure Pressure drops down the drillstring and through the bit play a part in BHP due to temperature effects. Temperature is also effected by: formation temperature influx of formation fluid (expansion of gas into the wellbore) Mechanical friction Pressure Harold Vance Department of Petroleum Engineering

Required injection rates???
Relating downhole air velocities to surface injection rates is quite complex. We need cuttings shape and size to determine terminal velocity Harold Vance Department of Petroleum Engineering

Minimum Energy Criteria
Probably the most widely used criteria was developed by Angel in 1957. Angel assumed that, for efficient cuttings transport downhole, the kinetic energy of the air striking each cutting should be the same as that of air giving efficient cuttings transport at standard pressure and temperature. Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering

Experience from shallow blast holes, drilled in limestone quarrying operations, indicated that cuttings were transported efficiently if the air velocity equaled or exceeded 3,000 feet per minute. This is equivalent to Gray’s terminal velocity for flat cuttings with a diameter of 0.46 in. or sub-rounded particles of 0.26 in. Harold Vance Department of Petroleum Engineering

Angel computed the downhole air pressure with eq. 2.5 Harold Vance Department of Petroleum Engineering

This was combined with the cuttings transport criterion defined in Eq 2.10 to deduce the minimum air flow rate as a function of hole depth, annular geometry, and penetration rate. Eq. 2.10 Harold Vance Department of Petroleum Engineering

To simplify, the average downhole temperature can be used to calculate BHP. This was solved numerically for the gas injection rate required to give an annular velocity equivalent in cuttings lifting power to air with a velocity of 3000 ft/min. A series of charts was generated for different geometries and penetration rates Harold Vance Department of Petroleum Engineering

Qmin can be approximated by: Qmin = Qo + NH Qo = injection rate (scfm) at zero depth that corresponds to an annular velocity of 3000 ft/min N = factor dependent on the penetration rate (Appendix C) H = hole depth, 1000 ft. Harold Vance Department of Petroleum Engineering

7-7/8” hole 3-1/2” drillpipe 6” drill collars 3800’ hole depth Harold Vance Department of Petroleum Engineering

Minimum BHP Criteria Angel’ analysis does not predict a minimum BHP, but gives a pressure that decreases monotonically with decreasing air flow rate. Harold Vance Department of Petroleum Engineering

Natural Gas Drilling Harold Vance Department of Petroleum Engineering

Terminal velocity of natural gas
Vtg = Vtair(1/S)0.5 Harold Vance Department of Petroleum Engineering

Natural gas drilling Lower density of natural gas than air results in: lower BHP lower drag forces Higher required circulation rates Non-ideal behavior of natural gas is not usually a problem since operating pressures are low and ideal behavior can be assumed. Harold Vance Department of Petroleum Engineering

Natural gas injection rate
A first order estimate can be derived by taking Angel’s figures for air drilling at the appropriate depth and penetration rate and dividing these by the square root of the gas’s specific gravity. Usually acceptable in practice Harold Vance Department of Petroleum Engineering

Mist Drilling Liquid volumes are only 1 to 2 percent at the prevailing temperature and pressure. Harold Vance Department of Petroleum Engineering

Hole cleaning, mist Water droplets act similarly to cuttings with slip velocity of near zero - mists do not clean the wellbore more efficiently than dry gas. Therefore annular velocities are high. Circulating fluid density is increased however and may add to the frictional pressure losses. Harold Vance Department of Petroleum Engineering

Hole cleaning, mist The increased density will lower the terminal velocity of the cuttings, but will increase the BHP reducing the volumetric flow rate at the bottom of the hole. Higher air injection rates are usually required when misting than with dry air. Harold Vance Department of Petroleum Engineering

Application of Angel’s method to mist drilling
Determine the penetration rate that would generate the same mass of cuttings as the mass of liquid entering the well over a time period. This includes any base liquid, foamer, and water influx. Harold Vance Department of Petroleum Engineering

Apparent equivalent ROP

Angel’s method for mist
The minimum air injection rate, required for good hole cleaning during mist drilling, is determined; either from Angel’s charts or from the approximation in equation 2.17 Harold Vance Department of Petroleum Engineering

Example Hole size = 7 7/8” depth = 5000’ Drillpipe size = 4 1/2” Anticipated ROP = 30 feet/hr Qo = 671, N = 65, H = 5000/1000 = 5 Minimum air rate for dry air = Qa = Qa +NH = x5 = 995 scfm Harold Vance Department of Petroleum Engineering

Example Liquid injection rate is 6 BPH Water influx is 3.8 BPH Total liquid rate is 9.8 BPH Penetration rate that would give this mass cuttings per hour is 60 ft/hr. Harold Vance Department of Petroleum Engineering

Example The minimum air rate required for dry air drilling at a penetration rate of 90 ft/hr using the value of N for 90 ft/hr, N = 98.3 would be 1162 scfm Harold Vance Department of Petroleum Engineering

Wellhead Design - Low pressure
Gas, mist, and foam drilling are normally utilized on low pressure wells Low pressure wells require simple wellhead designs Some operators opt for a simple annular preventer alone Harold Vance Department of Petroleum Engineering

However, a principal manufacturer of such equipment strongly cautions that such use exceeds the design criteria of this equipment. The minimum setup should consist of a rotating head mounted above a two ram set of manually-operated blowout preventers, consisting of a pipe ram and a blind ram Harold Vance Department of Petroleum Engineering

Slightly higher pressure systems should also have an annular preventer between the rams and the rotating head. For added safety the BOP system should be hydraulically operated Working pressure of these rotating heads is ~ psi Harold Vance Department of Petroleum Engineering

Wellhead Design - High pressure
Gasified liquids, flowdrilling, mudcap drilling Rotating heads on top of conventional hydraulically operated BOP usually suffice In Canada, nitrified liquids are often used with an RBOP installed atop a conventional BOP stack. Harold Vance Department of Petroleum Engineering

Blind rams should be installed in the bottom set of rams (when a two ram system is used) Sometimes a third set of rams (pipe rams) is utilized. In this case the RBOP is installed atop an annular preventer. The blind ram is placed between the two sets of pipe rams. Harold Vance Department of Petroleum Engineering

The lowermost set of rams should be installed directly atop the wellhead (or an adapter spool if necessary) You should never place any choke or kill lines below the lowest set of rams. If one of these lines cuts out, there is no way to shut in the well. Harold Vance Department of Petroleum Engineering

Care must be taken to utilize a rig with a substructure high enough so that the wellhead is not below ground level, with space enough to put the entire desired BOP stack below the rig floor Harold Vance Department of Petroleum Engineering

Wellhead Design - Snub drilling
Snub drilling and CT drilling have BOP stacks that allow tripping at much higher pressures than other forms of UBD (routinely up to 10,000 psi) Snubbing and CT units can be used for UBD at pressure that cannot be managed by conventional surface equipment. Harold Vance Department of Petroleum Engineering

Casing Design Casing design for UBD is not significantly different than conventional With air drilling, the casing tension should always be design with no buoyancy considered. No difference in burst design - usually Harold Vance Department of Petroleum Engineering

Casing Design Collapse design should always be based on an empty casing string A collapse design factor for UBD should be ~1.2 for UBD instead of (API design factor) Harold Vance Department of Petroleum Engineering

Casing Design - Corrosion control
For fluid filled wells, corrosion is usually not considered when drilling. Corrosion is not a factor when drilling with dry air Corrosion must be considered when drilling with mist, foam, or aerated fluids. Corrosion inhibitors should be added to the system Harold Vance Department of Petroleum Engineering

Casing Design - Casing wear
Casing wear is accelerated with gas drilling This is due to less lubrication by the drilling fluid Most air drilled holes are drilled faster and less time is spent rotating Doglegs add to casing wear Harold Vance Department of Petroleum Engineering

Completion Design If a well is properly drilled under underbalanced conditions, but is completed using overbalanced methods, much if not all of the impairment-reducing benefits might be permanently lost. Harold Vance Department of Petroleum Engineering

Underbalanced Completion Techniques
Running production casing, liners, slotted liners and other tools underbalanced. Controlled cementing of production casing or liners Running production tubing and downhole completion assemblies Perforating underbalanced Harold Vance Department of Petroleum Engineering

Running casing and liners underbalanced
If the completion is not open hole, casing or liners must be run Surface pressures are usually reduced by bullheading a heavier fluid down the annulus. This fluid may be more dense than that with which the well was drilled, but still must be light enough to prevent overbalance. Harold Vance Department of Petroleum Engineering

For casing and un-slotted liners, the well is usually allowed to flow while running the casing. This helps to prevent excessive surge pressures. A snubbing unit might be required to get the casing started in the hole. Harold Vance Department of Petroleum Engineering

Slotted liners do not allow the well to be shut-in when the liner is across the BOP stack. It may be necessary to flood the backside with drilling fluid to allow the running of the slotted liner into the wellbore Fluid is continuously pumped down the wellbore to reduce pressures Harold Vance Department of Petroleum Engineering

Cementing pipe underbalanced
If casing is run underbalanced, cementing should also be accomplished underbalanced. HSP of the cement slurry can be reduced by entraining gas, or by reduced density additives. Harold Vance Department of Petroleum Engineering

Running tubing underbalanced
No matter the production casing/liner design, production will almost always be required. With cemented casing and liners, the tubing can be run conventionally. Harold Vance Department of Petroleum Engineering

Running tubing underbalanced
Tubing can be run underbalanced in a number of ways snubbing CT diverting flow Setting a packer above the open zone with a temporary plug Harold Vance Department of Petroleum Engineering

Bit selection The bit selection process Assemble offset well data Develop a description of the well to be drilled Review offset well bit runs Develop candidate bit programs Confirm that the selected bits are consistent with the proposed BHA’s Perform an economic evaluation, to identify the preferred bit program Harold Vance Department of Petroleum Engineering

Assemble offset well data
Identify the nearest, most similar wells to the proposed location Gather as much information as possible about drilling these wells Include bit records, mud logs, wireline logs, daily drilling reports, mud reports, directional reports Harold Vance Department of Petroleum Engineering

Develop a description of the well to be drilled
Characterize the proposed hole geometry hole size casing points, trajectory Harold Vance Department of Petroleum Engineering

Outline the anticipated values of rock hardness and abrasivity at all depths Sonic travel time logs give qualitative indications of formation hardness. Low travel times - high rock compressive strengths Harold Vance Department of Petroleum Engineering

Outline the anticipated values of rock hardness and abrasivity at all depths Abrasivity is more difficult to quantify It is possible to form a qualitative assessment of the rock’s potential for abrasive bit wear. Abrasiveness is related to Hardness of its constituent minerals Bulk compressive strength Grain size distribution Shape Harold Vance Department of Petroleum Engineering

Make note of any formations that may have a special impact on bit performance Divide the well into distinct zones Each zone corresponds to a significant change in formation properties or drilling condition Harold Vance Department of Petroleum Engineering

Review offset well bit runs
Determine what bits were used to drill through each formation likely to be penetrated Identify which bit gave the best or worst performance Look at the bit grading Use the bit performance to infer formation hardness and abrasivity Harold Vance Department of Petroleum Engineering

Identify candidate bits
Identify which bits are candidates for each zone to be penetrated Consider fixed cutter and roller cone bits Harold Vance Department of Petroleum Engineering

Roller Cone Bits Key design considerations for roller cone bits are: cutting structure bearing seal types gauge protection Should be matched to the formations hardness and abrasivity Harold Vance Department of Petroleum Engineering

Fixed Cutter Bits Key design considerations for fixed cutter bits are: cutting structure body material and profile gauge stabilizing (anti-whirl) features Should be matched to formations hardness and abrasivity Harold Vance Department of Petroleum Engineering

Fixed Cutter considerations
PCD cutters wear rapidly in hard formations Impregnated and natural diamond bits tolerate very hard and abrasive formations Gauge protection is dependent on abrasiveness Harold Vance Department of Petroleum Engineering

Develop candidate bit programs
At this stage, develop several alternative bit programs. Consists of type of bit, start and end depths, and anticipated penetration rates. Harold Vance Department of Petroleum Engineering

Confirm that the selected bits are consistent with the proposed BHA’s
Do the operating parameters of the proposed BHA’s inhibit bit performance? Is WOB limited? Do the selected downhole motors exceed the rpm capabilities of the bits? Harold Vance Department of Petroleum Engineering

Perform an economic evaluation, to identify the preferred bit program
Use the estimated penetration rate and bit life to predict the probable cost for each bit run: Chi = CriTi + Cbi Predicted cost of the interval is the sum of all the bit costs for the particular bit program. Rank all the alternative bit programs Harold Vance Department of Petroleum Engineering

Bit selection for Dry Gas, Must and Foam drilling
Roller cone Fixed cutter Harold Vance Department of Petroleum Engineering

Roller Cone Bits Dry gas drilling produces a smoother hole bottom than with mud, and full coverage of the bottom of the hole with cutters is not as important. Larger teeth can be used for harder formations Abrasive wear is normally higher for dry gas drilling Harold Vance Department of Petroleum Engineering

Roller Cone Bits Cone offset is not as important with dry gas drilling Good gauge protection is very important Utilize sealed bearings Harold Vance Department of Petroleum Engineering

Fixed Cutter Bits PDC bits are usually a poor choice for dry gas drilling Not has heat tolerant Diamond bits may be heat tolerant. Harold Vance Department of Petroleum Engineering

Bit selection for gasified and liquid systems
Not much difference from conventional drilling Harold Vance Department of Petroleum Engineering

Underbalanced perforating
Can be performed with wireline or with tubing conveyed perforating guns. Harold Vance Department of Petroleum Engineering

Drillstring design Similar to conventional drilling There will be less buoyancy BHA should be designed so that all compression is in the BHA An exception is in horizontal wellbores. Harold Vance Department of Petroleum Engineering

Example 6 Harold Vance Department of Petroleum Engineering

Drillstring design Drillpipe is usually designed with: a design factor of 1.1 and an overpull from 50, ,000 lbf Harold Vance Department of Petroleum Engineering

Example 7 Harold Vance Department of Petroleum Engineering

Separator design Capacity is a function of:
Size Design and arrangement Number of stages Operating P and T Characteristics of fluids Varying gas/liquid ratio Size and distribution of particles Liquid level Well-fluid pattern Foreign material in fluids Foaming tendency of fluids Physical condition of separator Others Harold Vance Department of Petroleum Engineering

Maximum gas velocity Harold Vance Department of Petroleum Engineering

Gas Separating Capacity

Lesson 12 Well Engineering.

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