Petroleum Engineering Summer School Hole. H M Dubrovnik, Croatia. Workshop #26 June 9 – 13, 08 June 2008 GEOTHERMAL WELL DESIGN – CASING AND WELLHEAD Hagen Hole Geothermal Consultants NZ Ltd., Birkenhead, Auckland, New Zealand. • To counter losses of drilling fluid circulation ABSTRACT during drilling. The Geothermal well design process includes • Protection of the well and formation against consideration of the objectives and purpose of the well, erosion, corrosion, fracturing and breakdown. the subsurface conditions likely to be encountered during the drilling process, and the identification of In general, the shallower and outer casing strings are required equipment, materials, and drilling procedures necessary for the drilling operations, while the inner needed to ensure a satisfactory well completion and an strings are required for production purposes. The acceptable well life. drilling process follows a sequence of drilling to a certain depth, running and cementing a casing string, The design steps which are necessary to drill and establishing a wellhead (drilling or final), which allows complete a deep geothermal well safely are: the drilling of the next smaller diameter section to • Subsurface rock and fluid conditions. proceed. As a minimum two, but usually more, • Depths of casings and well completion. completely cemented, concentrically located, steel • Casing specification and cementing materials casing strings are obligatory both from a technical and and programmes legal sense for a geothermal well. • Wellhead specification Casing strings and liner for a typical geothermal is • Drilling fluids, drill string assemblies illustrated below in Figure 1. • The necessary drilling tools and equipment. Perhaps the most critical aspects of these design steps Connection to permanent is the selection of casings, casing specification, casing wellhead assembly shoe depths, and how the well is completed. This paper reviews the casing and wellhead specification process. Conductor Keywords: geothermal, well design, casing design and Surface Casing specification. INTRODUCTION Intermediate Casing (Not always necessary) The choice casing depths and specification of the materials weights and connections is vital to the Anchor Casing success and safety of the well drilling process and to the integrity and life of the well. All casings fully cement back The casing design and specification process includes to surface reviewing the required services of the casings, Production Casing 750 - 1500 m determination of the setting depths and checking possible failure modes. CASING SERVICES What is the purpose of the casing? The reasons for including casing strings and liners Slotted Liner include:- • Prevention of loose formation material from collapsing into and blocking the hole. NOTE: An intermediate casing • Provision of anchorage or support for drilling may be installed between the and the final wellhead. anchor and production casing. • Containment of well fluids and pressures. • Prevention of ingress or loss of fluid into or Figure 1. Casing strings and Liner for Typical from the well, and “communication” or Geothermal Well. leakage of fluids between different aquifers. 1 Hole. H M Petroleum Engineering Summer School Dubrovnik, Croatia. June 2008 Workshop #26 June 9 – 13, 08 In a hot water or two phase field with boiling CASING SETTING DEPTHS conditions as assumed above, it is possible (although The casing setting depths for a typical geothermal well unlikely) at any stage of the drilling for the well to be will be chosen from the following information and filled with a column of steam at a temperature and expectation related to the following aspects: saturation pressure corresponding closely to formation conditions at hole bottom, or at the level of greatest • Surface or Conductor Casings Strings – permeability. As this pressure is more than that of the These are the largest casings which are set at a formation fluid, there is a tendency for steam to escape shallow depth and are employed to prevent into upper permeable formations, and in weak loose near-surface material collapsing into the geological conditions blow out at the surface. Upper hole. They are also utilised to support the casing depths should beset to seal off possible leakage initial drilling wellhead, and to contain the paths to the surface and to limit the well fluid pressure circulating drilling fluid. The setting depth of at the shoe to that imposed by the overburden pressure, the casing shoe will estimated from geological or by the fracture gradient of the materials if this is deduction, but may be altered to reflect known. conditions found during the course of drilling, The competence of the rock and the incidence of and may have to contain hot fluid under drilling circulation fluid losses are likely to govern pressure if there is a thermal zone close to the casings depths, and thus the number of casing strings surface. needed to allow the target depth to be reached most economically. • Anchor or Intermediate Casing Strings – This ‘competence of the rock’ can only be derived These casings are intermediate in diameter from experience as suggested above, but usually falls and in setting depth which are set to support somewhere between a theoretically derived fracture successive wellheads (usually including the gradient and a theoretical overburden pressure. permanent wellhead) and to contain drilling Figure 2 below illustrates a theoretically based casing and formation fluids of relatively high shoe depth selection procedure. temperature and pressure. Setting depths will Casing Depths be chosen from expected formation rock and 0 fluid conditions to provide adequate Surface Casing Shoe permanent anchorage and additional security 200 Water Level against drilling problems including blowouts. Anchor Casing Shoe 400 • Production Casing – 600 This casing is smaller in diameter and set at greater depth than previous casings, and is Production used primarily to convey steam and water to 800 Casing Shoe the surface, but it is also important in facilitating drilling to total depth and to 1000 prevent unwanted leakage of fluids into or out 1200 pth (m) of different aquifers. The depth of this string Theoretical Overburden should be chosen first, on the basis of the De expected depths and temperatures of fluids to 1400 be included and excluded from production. 1600 Theoretical Fracture Gradient In the situation of appraisal or production drilling, the experience of earlier drilling and well testing in the 1800 area is the most useful guide in selecting casing depths. Steam in Well However, when drilling a first well in a new area, 2000 Water in reasonable assumptions must be made as to the Formation possible rock and fluid conditions to be expected down 2200 to the total drilled depth. These will be deduced from 2400 consideration of surface scientific surveys possibly 0 5 10 15 20 25 30 35 40 45 50 supplemented by the results of drilling a shallow Pressure (MPa) investigation hole at the site. In the absence of a clear understanding from the scientific data, it is frequently Figure 2. Example of Theoretical Casing Depth assumed that the reservoir fluid can be approximated to Selection. a column of water at boiling temperature throughout its This theoretical situation would require the production depth – ‘Boiling Point for depth (BPD). If the ground casing shoe being set at a depth of 800 m depth; the water level is known, the depth should be taken from anchor casing shoe being set at 300 m depth; and the below that datum. surface casing shoe being set at around 60 m depth. 2 Petroleum Engineering Summer School Hole. H M Dubrovnik, Croatia. Workshop #26 June 9 – 13, 08 June 2008 CASING DIAMETERS The design checks for axial stress can be separated into The diameters of the various strings of casing in any two sets of conditions – before and after the casing is well are chosen after consideration of the following cemented. aspects:- • Sufficient cross-sectional area to convey the Axial Loading before and during cementing expected / desired flow of fluid; Until the annular cement sets around the casing the • Sufficient annular clearances to run and tensile force at any depth includes the weight of the cement concentric casing strings; casing in air less the buoyant effect of any fluid in the • The use of casing sizes which are standard well. manufactured products which are readily available and match the handling tools usually T– held by drilling contractors. Thus: Fp = [ Lz Wp – (Lz-Lw)Ap/n]g Due to the manner in which different pipe thicknesses Where: are manufactured, tubular sizes are identified by their Fp = the tensile force at the surface from casing outside diameters and in accordance with the API weight specifications. Lz = depth of casing Wp = unit weight of casing Lw = depth of water level in well SERVICE CONDITIONS AND FAILURE MODES Ap = cross sectional area of pipe n = mean specific volume of hot fluid Whereas deep petroleum drilling considers the most g = acceleration due to gravity important parameters in casing design to be fluid pressure, casing weight, and tensile loading, in The design tensile force shall allow for the dynamic geothermal service generally the most severe service loads imposed during the running of the casing, which occurs as a result of high temperature loadings. The will include the drag force of the casing against the problem is compounded by facts that the service side of the well, particularly in a deviated well. This temperatures can seldom be predicted at all accurately; dynamic loading must be limited by specifying the and that the various types of casing steel grades and maximum hook load which may be applied. casing connections are manufactured specifically for petroleum service rather than geothermal service. In a deviated hole the maximum bending stress induced is:- The effects of elevated geothermal temperatures on F = EqD well components include:- b • Change in length of unrestrained pipe – for Where: example, 1.8 m expansion over a length of F = maximum stress due to bending 1000 m with a temperature change of 150°C.