Drilling Salt Formations Offshore With Seawater Can Significantly Reduce Well Costs
- Stephen M. Willson (BP America) | Peter M. Driscoll (BP America) | Arnis Judzis (TerraTek Inc.) | Alan D. Black (TerraTek Inc.) | J. Wesley Martin (TerraTek Inc.) | Brian L. Ehgartner (Sandia Natl. Laboratories) | Tom E. Hinkebein (Sandia Natl. Laboratories)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2004
- Document Type
- Journal Paper
- 147 - 155
- 2004. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.2.4 Risers, 2.4.3 Sand/Solids Control, 1.6.9 Coring, Fishing, 1.6.1 Drilling Operation Management, 1.6 Drilling Operations, 4.3.4 Scale, 1.14 Casing and Cementing, 2 Well Completion, 1.2.2 Geomechanics, 1.1.6 Hole Openers & Under-reamers, 1.10 Drilling Equipment, 2.1.7 Deepwater Completions Design
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The rate of penetration (ROP) increases significantly if salt formations are drilled with undersaturated fluids. This is especially true when drilling riserless. Of primary concern is the amount of hole enlargement that will occur. However, if managed, then a step change in drilling performance and costs can be achieved.
This paper presents in detail the results of a comprehensive and large-scale laboratory testing program performed on outcrop salt samples that replicate drilling salt formations with undersaturated drilling fluids with flow rates of up to 1,000 gal/min (gpm). The laboratory testing program includes tests performed in a computerized tomographic (CT) scanner to map hole enlargement in real time, as well as variously sized borehole leaching tests with borehole diameters of up to 6 in. Flow rates are scaled to produce comparable levels of turbulence occurring in the field.
An analytical hole-enlargement prediction model is presented that incorporates the effects of ROP, pump rate, drill-fluid saturation, dissolving salt drill cuttings, and salt leaching from the borehole wall. This model accurately predicts to within 10% the measured hole enlargements produced in the scaled laboratory tests for a wide range of flow rates and fluid saturations.
Predictions of field performance are made, and the implications of the predicted hole geometry are discussed. Provided that high rates of penetration are realized, acceptable hole geometries will result, even when pumping seawater at flow rates of up to 1,600 gpm. A field application is described whereby the historical practice of drilling the last 200 ft of the 20-in. hole into salt using a saturated brine drilling fluid was discontinued in preference to the continued use of seawater. The competing influences of increasing rate of penetration and avoiding the cost of a sacrificial mud system, offset by the increased cost of cement, resulted in cost savings of U.S. $250,000 per well. Implemented in a multiwell subsalt development, cost savings of more than U.S. $1.5 million will be realized in the drilling program. Further applications of this technology are now being sought.
Significant thicknesses of salt underlie the Gulf of Mexico (GOM) and other hydrocarbon basins in the world. In the GOM, particularly, salt may be encountered within a few thousand feet of the mudline. Typically, the salt bodies penetrated range in thickness between 1,000 and 10,000 ft. At these shallow depths, current deepwater drilling practice is to drill riserless, with returns (the drilling fluid used and the formation cuttings) being discharged to the seabed. This is adopted because of the large-diameter hole sections needed at shallow depth to accommodate the multiple casing strings required to reach the deep reservoir formations. To take returns to the drill floor, a marine riser is required. Current drilling technology and required pressure ratings limit the riser diameter to 18 3/4 in. when drilling in overpressured environments. This means that hole diameters greater than this (e.g., the 28- and 24-in. hole sections) have to be drilled riserless.
When drilling in shallow formations, it is common practice to use seawater as the drilling fluid and to use viscosified pills of polymerized brine to ensure effective hole cleaning. However, where the potential for shallow-water flows occur, weighted brines of the required density are used to limit brine and uncemented sand influx to the wellbore because this can compromise well and subsea template integrity. When drilling salt sections riserless, there is a concern that the undersaturated brine or seawater drilling fluid will cause severe enlargement within the salt because it is leached (dissolved) by the less saline fluid. This has led to shallow salt sections being drilled with either salt-saturated or high-salinity, water-based drilling fluids, with returns being taken to the seabed.
There are several disadvantages with this current salt-drilling practice. Because of the large hole diameters being drilled (28- and 24-in. diameters are typical at the depths that salt is often encountered), high circulation rates of up to 1,000 gpm (23.8 bbl/min) are often used. The ROP is also slowed using weighted, high-salinity brine drilling fluids; 20 to 30 ft/hr is not uncommon in these instances. Under these conditions, a 1,000-ft-long hole section in salt, drilled riserless, would discharge approximately 50,000 bbl of drilling fluid onto the seafloor. Also, there are cost implications of adopting this drilling strategy. The cost of the weighted brine alone that is lost when drilling riserless could be in excess of U.S. $500,000. If ROPs were increased by a factor of three, then a similar amount of additional savings could be realized by drilling the well faster with the use of expensive "fifth-generation" deepwater drilling rigs. There is a significant cost benefit, therefore, in identifying alternative drilling fluids when drilling salt sections riserless. The cheapest of these is seawater.
Review of Salt Drilling Experiences
The most recent compilation of GOM salt drilling experiences has been that of Whitfill et al., 1 "Drilling Salt-Effect of Drilling Fluid on Penetration Rate and Hole Size." This paper, published in 2002, covered several aspects of drilling salt, including drilling with undersaturated brine and the use of undersaturated sweeps.
They reported that field experimentation into the use of low-viscosity undersaturated sweeps while drilling salt with an otherwise saturated water-based drilling-fluid system resulted in a doubling of the ROP. While drilling with a tricone bit in a 17-in. borehole, two 100-bbl seawater sweeps viscosified with 0.75 lbm per barrel (ppb) Xanthan gum were pumped. These temporarily increased the ROP from a nominal 25 ft/hr to 60 ft/hr and 49 ft/hr, respectively. Later, 500-bbl sweeps were pumped; these were weighted to 11.2 ppg and contained dispersants and salt to a chloride level of 150,000 parts per million (ppm). Typical temporary increases in ROP were from 21 to 41 ft/hr; 30 to 59 ft/hr; 15 to 20 ft/hr; and 51 to 91 ft/hr. This experience indicates that significant increases in ROP are achievable when drilling with seawater.
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