Basic Casing Design and Casing Point Selection Ashley Fernandes
Objectives On completion of this module the engineer should be able to… Define the different types of casings. Describe the casing point selection process Describe the maximum load casing design method. Demonstrate the bottom up method of casing setting depth initial estimation.
3 AJF Introduction Design Considerations Cost, (usually up to 17% of the total well cost) Bottomhole Pressure, Service Conditions (casing handling) Material Properties Internal Yield Collapse Tension
Statement of Standard All casing and tubing's shall be designed to withstand all loads that can be imposed on them during installation and the lifetime of the well. No well construction program shall be commenced without an approved casing and tubing design.
5 AJF Mechanical Properties Of Steel API Standards Specification; API, 5A. Bulletins; 5C2, properties of casing, tubing, drill pipe. Recommended; API. RP7G, care and use of tubular. H2S & CO2 Exposure to more 0.05 psi of H2S pressure and CO2 corrosion can lead to failure, Common practice is the use chromium alloy (casing type L80 or Stainless steel)
Types of Casing Strings Conductor Surface Intermediate Drilling Liner Production
Types of Casing Strings Conductor Casing Purpose Provides mud returns to tanks. Divert flow in case of emergency. Support subsequent casing loads. Installation Driven Rathole set Jetted Drilled and cemented
Types of Casing Strings Surface Casing Purpose Protect Fresh water aquifers. Provide wellbore integrity, - Provides a BOP seal - Allows drilling into abnormal pressure safely Definition: Casing set at or above 6500’ or in sub-normal pressure. Setting depth is based on mechanical and regulatory considerations
Types of Casing Strings Intermediate Casing Purpose Provide mechanical integrity. - Case off problem zones. - Provide higher fracture gradient as shoe for drilling operations and provide contingencies such as well control. Definition: Casing set to 6500’ or deeper in abnormal pressure. Setting depth is based on mechanical considerations
Types of Casing Strings Drilling Liners Purpose Provide mechanical integrity as intermediate casing but at lower cost. Definition: Partial string of casing hung in previously set casing string and is set to a depth greater than 6500’ or in abnormal pressure. Setting Depth is based on mechanical considerations
Types of Casing Strings Production Casing/Liners Purpose Provide isolation of the producing zone from other zones. Withstand the anticipated loads during production and testing operations for the wells life time. Setting Depth is the last string across the production zone. This may be a casing to surface or a liner that is hung.
12 AJF Casing Point Selection Why Casing is Run in the Hole? Casing is set for two Drilling reasons; To consolidate the hole already drilled, (steel filter cake) To provide the pressure control integrity to drill ahead.
Casing Point Selection Criteria Mechanical Considerations Ability of the weakest exposed formation beneath the casing to withstand the load imposed by well control operation. Likelihood of differential sticking occurring while running casing.
14 AJF Criteria For Selecting Casing Depths
15 AJF Other Restrictions On Casing Shoe Depth Within the limit allowed by kick tolerance, we may restrict the length of OH sections, to minimize deterioration of wellbore with time Formations may affect casing depth (reactive shale) Directional Problems may alter casing points (drag, torque)
Casing Point Selection Criteria Other factors affecting casing setting depth. Underground fresh water zones Shallow hazards Directional profile S/T requirement Aquifers to case off ECD at shoe
17 AJF Special Criteria for Conductor The Conductor pipe needs to be set deep enough to carry subsequent axial loads from all other casing strings. It also must withstand the bending loads from environmental conditions.
18 AJF Special Criteria for Surface Casing The Surface Casing is usually set in the first competent formation which is strong enough to close in on a kick.
19 AJF Special Criteria for Intermediate Casing
20 AJF Intermediate Casing - other considerations Reduce torques and drags in an extended reach hole. Case off possible differential sticking zones and perform directional work below casing. Case off some problem zones prior to drilling ahead. Intermediate casing may also be set for directional or wellbore stability reasons
21 AJF Special Criteria for Production Casing The Production Casing is set through or just above the reservoir, depending on the type of completion to be used.
Casing Point Selection Criteria Information gathered prior to casing design. Estimated pore pressure and rock strength using offset data. The minimum and maximum casing sizes to be run at TD that would allow for logging testing and a completion program. The effects of geological uncertainties on casing setting depths and the ability to safely circulate out the maximum anticipated kick volume
Casing Point Selection Criteria After gathering Information Develop a pore pressure and fracture gradient versus depth plot. Plan the well from TD up. Determine the maximum formation pressure at TD. Add a Trip margin and determine minimum weight at TD.
Casing Point Selection Criteria
Bottom Up Method Plot the pore pressure gradient curve Plot the Mud weight curve. The mud weight should balance the highest PP in the OH with a TM of 0.5 ppg. Plot the estimated actual FG curve, and the designed FG curve, which is FG less allowance for Well control, surge or ECD. Start on the bottom on the mud weight curve and draw a vertical line up to the designed FG curve. This is the initial estimated production casing or liner. Cont…..
26 AJF Kick Tolerance . Kick tolerance is a measure of the size of a gas kick that can be handled. The following assumption are made… A gas influx from TD at the casing point or at TD. The kicking formation has a pore pressure equal to or greater than the mud hydrostatic. Shut in casing pressure = MAASP when the top of gas is at the casing shoe, using the drillers method. Based on these assumptions, we calculate the volume of a gas kick. This is the maximum size of gas influx, which is what we call KICK TOLERANCE
27 AJF Kick Tolerance More assumptions For an Exploration or Appraisal well, we can assume that the kicking formation may have a pore pressure gradient of 10% higher than the mud gradient. In this case a planned mud gradient of 0.5 psi/ft will assume a pore pressure gradient at the kick depth of 0.55 psi/ft. For a development well in a known area, assume that the kicking formation may have a pore pressure which is equal to the mud gradient. In this case any kick taken will be a swabbed kick.
28 AJF Kick Tolerance Example: Casing shoe at 6000’ with a FG of 0.72 psi/ft, plan to drill to the next casing point at 8500’ with a mud gradient of 0.62 psi/ft in a vertical exploration well. (Kick gradient 10%more than mud gradient.) Assume a gas gradient of 0.12 psi/ft at the casing shoe, and 12 ?” hole with 5” DP and 300’ of 8” drill collars.
29 AJF Kick Tolerance Example2: Casing shoe at 5000’ with a FG of 0.65 psi/ft, plan to drill to the next casing point at 7500’ with a mud gradient of 0.55 psi/ft in a vertical exploration well. (Kick gradient 10%more than mud gradient.) Assume a gas gradient of 0.12 psi/ft at the casing shoe, and 12 ?” hole with 5” DP and 300’ of 8” drill collars.
Maximum Load casing Design This is a method for selecting specific casing based on operational conditions and resulting stresses. Concept: Design for most severe realistic load to minimize risk of failure. Maximum load cases are based on geographical region, geologic section and organizational philosophy.
Maximum Load casing Design Design Cases: Burst Collapse Tension Special considerations: H2S/CO2, Temperature, Multi-axial stress correction, Stability.
Steps for Basic Casing Design Develop Maximum Load case condition for burst Calculate resulting loads, (Design Line) Select casing strings with load capacity >/= Load line Burst Design Collapse Design Develop Maximum Load case condition for Collapse For casing selected in burst design, check that load capacity >/= Load line Develop Maximum Load case condition for Tension Tension Design Calculate resulting loads, (Design Line) Calculate resulting loads, (Design Line) Multiply design line by safety factor, (Load Line) Multiply design line by safety factor, (Load Line) Multiply design line by safety factor, (Load Line) For casing selected in collapse design, check that load capacity >/= Load line
Minimum Design Factors
Casing Burst Design Maximum pressure at casing shoe,dependent on FG. (Internal Pressure) The maximum internal burst loading pressure at surface is a function of the injection pressure, and is given as: Load Line External pressure due to annular drilling fluid….0.465psi/ft Worst case scenario is gas filled above FG+SF
Casing Burst Design Pressure with burst design factor of 1.1 Design Line @ shoe @ surface @ shoe @ surface From the casing Tables choose appropriate casing
Casing Collapse Design Load Line Worst case scenario is full evacuation with Mud on the outside Maximum collapse pressure at shoe, (external pressure) The casing is empty and open to atmospheric pressure. This will render the surface pressure to be 0 psi Design Line From the casing Tables check casing collapse rating
Casing Tension Design Worst case scenario is stuck casing with an over pull 100,000 lbs applied As per SLB casing manual, TVD should be used for gravity related Tensile load calculation. If Biaxial stress calculations are not being Made then a safety factor of 1.6 shall be used. Load Line Design Line From the casing Tables check casing tensile rating Tensile load will change if pumps are on while pulling on casing.
38 AJF Casing Sizes Decision Tree Tubing size, in Casing & liners size, in Bit & hole size, in Casing size, in Casing & liners size, in Bit & hole size, in Casing & liners size, in Bit & hole size, in Casing size, in Bit & hole size, in 16 14-3/4 11-3/4 11-7/8 10-5/8 8-5/8 7-7/8 5-1/2 5-3/4 4-1/2 3-1/2 1.9 20 17-1/2 13-3/8 14 12-1/4 8-3/4 6-1/8 4-1/2 2-3/8 10-3/4 9-1/2 7-5/8 7-3/4 6-1/2 5 24 20 16 14-3/4 11-3/4 11-7/8 10-5/8 8-5/8 7-7/8 5-1/2 2-7/8 30 26 20 17-1/2 13-3/8 14 12-1/4 9-5/8 8-1/2 8-3/4 7 3-1/2 18-5/8 9-5/8 9-7/8 8-1/2 7 5-7/8 6-5/8 4-3/4 4 2-1/16
39 AJF API CASING SPECIFICATIONS API Specifications (5A, 5AC and 5AX) Weight of Casing Nominal Weight Expressed in lb/ft or kg/mt ( Weight of CSG plus tool joint ). Plain End Weight Expressed in lb/ft or kg/mt ( Weight of CSG w/out tool joint) . Length of Casing Range Length (ft) Average length (ft) 1 16 -25 22 2 25 -34 31 3 Over 34 42
40 AJF API CASING SPECIFICATIONS Colour Grade Identifications