Solid Expandable Tubular
Standard Well Design Process: The standard well design process starts bottom up. The reservoir pressure and production flow requirements define the size of the tubing required to flow the well. The production casing or liner should have an inside diameter that allows adequate radial clearance between the tubing and casing to allow for running and fishing of the tubing if required. Once the size of the production casing is established, the size of the production hole to be drilled is decided. The bit size must be 0.375”-0.5” or preferably 0.75” larger than the OD of the planned production casing or liner string. Enough hole clearance is required to allow the deposition of mud cake while drilling and installing centralizers on the casing pipes for proper centralization and effective cementation.
Once the required size of the production hole is established, the second last casing size is decided ensuring that it has the required drift diameter for the bit to be able to pass through the casing for drilling the production hole. The casing size/bit size chart is used to select the required sizes for casing and bit to be able to meet these objectives. The casing-bit selection chart provides size options and some flexibility in selection. This process continues upward till reaching the surface casing size and bit size selection. The casing seat selection and casing strength design calculation based on pore pressure-fracture pressures & mud weight requirement indicates how deep each casing string can be run maintaining required design safety limits.
Benefits of Expandable Tubular: As you can very well envision, following the standard well design process could limit how deep the well can be drilled. Many a time, unexpected formation troubles during drilling force engineers to short-land some of the casing strings, which enforces further limits on how deep the well can safely be reached.
The well design using standard casing pipes and sizes poses mainly two limitations:
Requiring too many casing strings to safely handle conditions like weak zone, depleted zone, shale issues, etc. If there are such zones in short intervals, a separate casing or liner string may need to be run to cover such zones before drilling ahead safely.
Unexpected well conditions requiring short landing of casing string before reaching planned casing depths. If a string needs to be short-landed, it will require either extending the next hole section, which could make it difficult to maintain the mud weight within the pore pressure - fracture pressure window, or limiting the target depth that can be reached before running out of casing sizes.
These issues can be addressed to a certain extent by using flush joint casing (without collar), special clearance casings, and using odd-size bits or an under reaming strategy. The invention of Solid Expandable Tubulars in the late 1990s expanded the horizon by providing more options to handle these well design complications. Unlike regular casing joints, expandable tubular is expanded downhole after running to planned depth. That provides the advantage of having enough running clearance within the previous casing string and effectively extending the depth of the current hole section.
Technical information: Expandable tubulars are not available in standard API casing weight and grades. Their pre and post-expansion properties are specified by the manufacturer. Since the pipe goes through elastic-plastic deformation, the material needs to be ductile enough to deform and strong enough to maintain the required strength after expansion. Hence the manufacturer-specified burst, collapse, and tensile strengths need to be considered and evaluated if they meet well pressure and design criteria. Technological evolutions have made it possible to manufacture high-strength expandable tubular, further expanding the horizon and their application. The upper part of the expandable pipe string contains a set of seals, which flush with the previous casing, ensuring annular isolation after expansion.
The pipe can be expanded in two ways:
Using a swage or a roller. A swage mandrel is run with the pipe and then pulled bottom-up with hydraulic and mechanical force to expand the pipe to the mandrel’s diameter. Using a swage ensures the application of uniform hoop stress and results in uniform thickness of the expanded pipe. However, if the swage gets stuck halfway through due to operational reasons or equipment breakdown, the only way to recover from the situation will be by milling the mandrel.
Rollers have been used for casing patches or repairing partially collapse casings much before the advent of expandable tubular. Rollers expand the pipe by rotating from top to bottom but have the disadvantage that they result in non-uniform expansion and uneven pipe thickness.
Since the expandable tubular is run as standard casing and then expanded to achieve the required drift diameter, the hole needs to be under-reamed to accommodate pipe expansion and have enough cementing clearance. As a standard process, cement is displaced in the annulus of the expandable tubular before it is expanded.
Caution points: Solid Expandable Tubulars is a fast-evolving but complex technology and hence should be planned well in advance. The collapse rating of expanded pipe is usually less than similar size conventional casing due to thinner walls and possible non-uniform expansion. If the expandable casing is planned as part of the well design, it helps in organizing required equipment and services beforehand and carrying out a proper risk assessment to address anticipated challenges. Even if it is considered as a contingency measure to be deployed depending on well conditions, including it in the well plan and design as a contingency option makes it much easier to utilize this option if the situation arises. Pre-planning for drilling, casing loads, safety factors, and cement design is highly recommended to minimize unexpected issues whether deploying the expandable tubular as a planned or as a contingency option.