Casing design
To design a casing string, one must have knowledge of:
Purpose of the wellGeological cross sectionAvailable casing and bit sizesCementing and drilling practicesRig performanceSafety and environmental regulations
To arrive at the optimal solution, the design engineer must consider casing as a part of a whole drilling system. A brief description of the elements involved in the design process is presented next.
Design objective
The engineer responsible for developing the well plan and casing design is faced with a number of tasks that can be briefly characterized.
Ensure the well's mechanical integrity by providing a design basis that accounts for all the anticipated loads that can be encountered during the life of the well.Design strings to minimize well costs over the life of the well.Provide clear documentation of the design basis to operational personnel at the well site. This will help prevent exceeding the design envelope by application of loads not considered in the original design.
While the intention is to provide reliable well construction at a minimum cost, at times failures occur. Most documented failures occur because the pipe was exposed to loads for which it was not designed. These failures are called "off-design" failures. "On-design" failures are rather rare. This implies that casing-design practices are mostly conservative. Many failures occur at connections. This implies that either field makeup practices are not adequate, or the connection design basis is not consistent with the pipe-body design basis.
Design method
The design process can be divided into two distinct phases.
Preliminary design
Typically the largest opportunities for saving money are present while performing this task. This design phase includes:
Data gathering and interpretationDetermination of casing shoe depths and number of stringsSelection of hole and casing sizesMud-weight designDirectional design
The quality of the gathered data will have a large impact on the appropriate choice of casing sizes and shoe depths and whether the casing design objective is successfully met.
Detailed design
The detailed design phase includes selection of pipe weights and grades for each casing string. The selection process consists of comparing pipe ratings with design loads and applying minimum acceptable safety standards (i.e., design factors). A cost-effective design meets all the design criteria with the least expensive available pipe.
Required information
The items listed next are a checklist, which is provided to aid the well planners/casing designers in both the preliminary and detailed design.
Formation properties: pore pressure; formation fracture pressure; formation strength (borehole failure); temperature profile; location of squeezing salt and shale zones; location of permeable zones; chemical stability/sensitive shales (mud type and exposure time); lost-circulation zones, shallow gas; location of freshwater sands; and presence of H2S and/or CO2.Directional data: surface location; geologic target(s); and well interference data.Minimum diameter requirements: minimum hole size required to meet drilling and production objectives; logging tool outside diameter (OD); tubing size(s); packer and related equipment requirements; subsurface safety valve OD (offshore well); and completion requirements.Production data: packer-fluid density; produced-fluid composition; and worst-case loads that might occur during completion, production, and workover operations.
Other: available inventory; regulatory requirements; and rig equipment limitations.
Preliminary design method
The purpose of preliminary design is to establish:Casing and corresponding drill-bit sizesCasing setting depthsThe number of casing strings
Casing program (well plan) is obtained as a result of preliminary design. Casing program design is accomplished in three major steps:
Mud program is preparedThe casing sizes and corresponding drill-bit sizes are determined
The setting depths of individual casing strings are found
Mud program
The most important mud program parameter used in casing design is the "mud weight." The complete mud program is determined from:
Pore pressureFormation strength (fracture and borehole stability)LithologyHole cleaning and cuttings transport capabilityPotential formation damage, stability problems, and drilling rateFormation evaluation requirementEnvironmental and regulatory requirements
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