PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 1 The Majority of Problems in Drilling are Somehow Related to the Mud Program PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 2 Under-Balanced Drilling Underbalanced Drilling is a drilling technique where the hydrostatic pressure of the drilling fluid is less than the formation pressure. Formation fluids flow into the wellbore during underbalanced drilling. Under-Balanced Drilling is also referred to as Managed-Pressure Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 3 Underbalanced Drilling • Overbalanced condition: 𝑃𝑓 < 𝑃hyd • At balanced condition: 𝑃𝑓 = 𝑃hyd • Underbalanced condition: 𝑃𝑓 > 𝑃hyd PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 4 Underbalanced Drilling UBD Techniques • • • • Light-weight Drilling Fluids Gas Injection down the drillpipe Annular Gas Injection via parasite string Foam or aerated drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 5 Underbalanced Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 6 Underbalanced Drilling Light-weight Drilling Fluid Technique • Light-weight drilling fluids such as water (freshwater or saltwater), crude oil, diesel are the most common techniques to achieve UBD. • In normally pressured areas with 8.65 ppg pressure gradients, it may not be possible to reduce the hydrostatic pressure to be less than formation pressure. • Using MW less than formation pressure is a very common UBD technique. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 7 Underbalanced Drilling Gas Injection down the Drill Pipe This technique requires compressors and Underbalanced Drilling (UBD) is achieved by injecting gas into the mud discharge line and aerate the mud. Air and nitrogen can be used to aerate the mud and reduce its density. Introducing air or oxygen into a hydrocarbon environment can cause fire and corrosion issues. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 8 Underbalanced Drilling Annular Gas Injection The annular drilling fluid can be aerated by running a parasite string or spaghetti string into the annulus. This procedure is more expensive than gas injection into the Drill Pipe due to increased rig time to handle more pipe. It does however maintain constant pressure on the bottom of the hole. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 9 Underbalanced Drilling Foam Drilling Foam Drilling is a common technique for UBD. Nitrogen is the gas used to create the foam. These foams can be very stable and safe to use. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 10 Underbalanced Drilling Advantages • Increased penetration rate • Elimination of Formation Damage due to mud filtrate invasion. • Reduced risk of differential sticking of pipe • Minimized and/or eliminated lost circulation while drilling • Improved formation evaluation • Increased well productivity PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 11 Advantages of Underbalanced Drilling Increased Penetration Rate Rate of penetration increases with reduced MW. Rock removal is easier and faster with less hydrostatic pressure at the bottom of the hole. The formation pressure is greater than the hydrostatic pressure of the drilling fluid with aerated drilling fluid. In an underbalanced condition, the formation pressure actually assists in rock removal from underneath the bit. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 12 Advantages of Underbalanced Drilling Elimination of Formation Damage due to mud filtrate invasion With the formation pressure greater than the wellbore pressure of the drilling fluid, mud filtrate cannot flow into formation. Instead, formation fluids flow into the wellbore. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 13 Advantages of Underbalanced Drilling Reduce the risk of Differential Sticking Differential Sticking is caused by wellbore pressure that is greater than formation pressure “pushing” the drill string against the side of a permeable formation in the wellbore. This cannot occur if the formation pressure is greater than the pressure in the wellbore. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 14 Advantages of Underbalanced Drilling Minimize and / or Eliminate Lost Circulation Issues While Drilling Weak formations can break-down when the hydrostatic pressure of drilling fluids exceed the fracture pressure of the rock. This results in the drilling fluids flowing into that formation instead of circulating back to surface with the cuttings. Aerated Drilling Fluids can be used to drill through Lost Circulation formations. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 15 Advantages of Underbalanced Drilling Increased Well Productivity This is related to Elimination of Formation Damage in that some formations are “water sensitive” due to clays, minerals, etc. Water from FWBM filtrate invasion can cause these shales and minerals to swell which will cause a reduction in porosity and permeability in the near-wellbore region. Underbalanced drilling can prevent this from occurring by preventing the water filtrate from contacting the formation. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 16 Underbalanced Drilling Disadvantages Safety – the well is essentially in a blow-out condition while drilling underbalanced. Wellbore instability problems – sloughing, caving, etc Water influx issues – mud contamination Downhole fires (if air is used to aerate the drilling fluid) Directional / horizontal drilling equipment – more expensive Economic factors - same Environmental Issues regarding the disposal of oil and/or saltwater contaminated mud. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 17 Disadvantage of Underbalanced Drilling Wellbore Instability Issues With the formation pressure greater than the wellbore pressure, Unconsolidated Formations can slough into the wellbore and prevent further penetration as well as stuck pipe. The solution to drilling with underbalanced fluid through unconsolidated formations could require setting additional casing strings to seal-off the sloughing formation. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 18 Disadvantage of Underbalanced Drilling Water Influx Permeable water-bearing formations are able to flow into the lower-pressure wellbore. If the rate of water influx is great enough, air drilling or aerated drilling will be “drowned-out” and conventional drilling fluids will be necessary to continue drilling operations. If UBD is being achieved by using lightweight drilling fluids or reduced mud weight, the water influx most likely will be salt water. This causes salt contamination and disposal issues with the drilling mud. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 19 Disadvantage of Underbalanced Drilling Downhole Fires Introducing oxygen into a hydrocarbon environment can result in unpleasant situations including corrosion and downhole fires. Air is not a wise choice of gas to use when using gas injection techniques to achieve UBD. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 20 Disadvantage of Underbalanced Drilling Directional and Horizontal Drilling Air is compressible therefore MWD tools that rely on mud pulse telemetry cannot be used. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 21 Disadvantage of Underbalanced Drilling Economic Factors Directional and Horizontal UnderBalanced Drilling is usually more expensive than vertical UnderBalance Drilling. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 22 Underbalanced Drilling Safety Issues Underbalanced drilling is working on a live well that is constantly in a blow-out condition. Well control must be maintained. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 23 Good Candidates for UBD • Pressure depleted formations • Areas prone to differential pressure sticking • Hard rock (dense, low permeability, low porosity). • “Crooked-hole” country and steeply dipping formations PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 24 Good Candidates for UBD • • • • • Fractured formations Vuggy formations High permeability formations Highly variable formations Once the optimum candidate has been identified, the appropriate technique must be selected, based on much of the same data required to pick the candidate PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 25 Density of Various Drilling Fluids PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 26 Underbalanced Drilling Techniques • Air Drilling Gases Dry air Nitrogen Carbondioxide Natural gas • Aerated Fluids Air-liquid mixtures (gasified liquids) Mist • Foams • Emulsions PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 27 Underbalanced Drilling Fluids 6.7 ppg 5 ppg 8.33 ppg 1.7 ppg 10.0 ppg PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 28 Underbalanced Drilling Flow Patterns LIQUID AERATED LIQUID PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 FOAM MIST GAS 29 Underbalanced Drilling Equipment • Rotating head • Closed-pressurized surface system • Separator • Compressor PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 30 Rotating Head PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 31 Rotating Head PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 32 Underbalanced Drilling Equipment Rotating Head • A Rotating Head is used to permit drilling under pressure. The Rotating Head has a rotating, steel reinforced rubber element that seals around the drill pipe or kelly. A diverter below the seal causes the returning drilling fluid to be routed to either the mud return line or to the choke manifold. • The sealing elements rotate with the Drill Pipe or Kelly to minimize wear PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 33 Underbalanced Drilling Equipment Rotating Head Iron Horse Tools, LLC Iron Horse – 5 PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 34 Underbalanced Drilling Equipment • • • • Closed-pressurized surface system Separator Compressor MWD tools PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 35 Air Drilling • Air/Gas drilling (also called “dust drilling”) is a technique used in areas where the formations are “dry” i.e., there is no influx of water or liquid hydrocarbons. • Also used to drill conductor hole / surface hole in areas with thick, hard limestone caprock at the surface. • This medium requires significant compressed gas volumes to clean the well with average velocities of over 3,000 ft per minute • Air Drilling uses percussion type bits that pulverize the rock and the recovery of samples for analysis is difficult, at best. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 36 Air Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 37 Air Drilling Applications • The most common applications for Air Drilling are: 1. In hard, dry rock drilling where less than 15 ft/hr ROP’s are obtained using conventional drilling techniques. 2. Crooked hole problems. 3. Lost Circulation areas. 4. Water sensitive formations. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 38 Air Drilling Disadvantages • Disadvantages to Air Drilling include: 1. Air Drilling can only be conducted in relatively “dry” rock. Water influx from formations can “drown” circulation. 2. Downhole and surface fires due to adding oxygen to hydrocarbon and creating an explosive mixture. 3. Sloughing due to the absence of hydrostatic pressure to maintain wellbore stability. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 39 Air Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 40 Air Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 41 Mist Drilling • Mist is created by the addition of 6 to 30 bbl/hr of fluid to the gas stream. The added fluids are typically water, surfactant and inhibitors. • The mist cleans, cools and lubricates the bit. • Cuttings are transported to the surface as a mist or more normally in a modified two phase flow. • Mist Drilling has slower ROP than Air Drilling but higher than conventional drilling. • If excess water influx is encountered, it will be necessary to change to either Foam, Aerated or Conventional Drilling Techniques. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 42 Foam Drilling • Water, surfactants and gas are combined to create a stiff foam that is used for the drilling fluid. The Cuttings-Carrying Capacity is up to 7 times greater than Air Drilling. • Foam is the most versatile of the gas-generated systems • Effective operating range from 0.2 to 0.6 s.g. • Mixture of gas phase and foaming solution • Foam flow varies with depth in the hole • An adjustable effective BHP is possible. • The disadvantage to Foam Drilling is that the returning foam requires time and sometimes chemical breakers to settle. Large reserve pits are required to contain the foam while it “breaks”. PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 43 Foam Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 44 Foam Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 45 Foam Drilling PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 46 Aerated Fluids • Aerated Drilling Fluids are created by gasifying the Primary Drilling Fluid. This is accomplished by adding air or nitrogen into the drilling fluid. • Methods Standpipe injection at surface Jet Sub above the bit Parasite String • Most drilling fluids can be aerated however nitrogen must be used when aerating OBM. Corrosion inhibitors must be added when air is the gas being used. • Aerated Drilling Fluids were initially designed as a technique to lighten mud to reduce lost circulation • As an UBD fluid, it is easiest to control in small diameter holes PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 47 Aerated Fluids PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 48 Aerated Fluids PE3043, Drilling Engineering I / Mike Stafford, PE Spring, 2018 49