The most expensive problem routinely encountered in geothermal drilling is lost circulation, which is the loss of drilling fluid to pores or fractures in the rock formations being drilled. Lost circulation represents an average of 10 percent of total well costs in mature geothermal areas, and often accounts for more than 20 percent of the costs in exploratory wells and developing fields. Well costs, in turn, represent 35 percent to 50 percent of the total capital costs of a typical geothermal project. Therefore, roughly 3.5 percent to 10 percent of the total costs of a geothermal project can be attributable to lost circulation.
This loss is harmful for several reasons, and the tendency toward lost circulation is aggravated by the pressure imbalance between the relatively cool (denser) column of drilling fluid and the hot (lighter) geothermal fluids in the formation.
If the drilling fluid fails to clean the hole and return cuttings to the surface, the cuttings can fall back onto the bottom-hole assembly and stick the drilling assembly.
Drilling fluid – especially in many high-temperature formulations – is expensive, and losing it to the formation instead of re-circulating it is costly.
In geothermal wells, the production zone usually is a lost-circulation zone, so it can be difficult to cure a harmful lost-circulation zone while preserving its productive potential.
Lost circulation can lower the fluid level in a well suddenly. Decreasing the static head of drilling fluid in a hot formation can allow the formation fluids to enter the wellbore, causing a loss of well control. This can occur either in productive or non-productive zones.
In the intervals that are not to be produced, the lost-circulation zone must be sealed to provide a wellbore that can be cased and cemented to the surface, or the cementing process must accommodate getting a good cement job when a lost-circulation zone is present. Adequately cementing the casing through lost-circulation zones is a major problem and a major cost.
Placement of lost-circulation materials (LCM) is difficult because the top and bottom of the loss zone often are not well known. The LCM or cement being used to heal the loss zone are especially likely to migrate away from the targeted placement zone if drilling has continued well past it into another loss zone, or if there is considerable rat hole below the original loss zone.
In many areas where geothermal drilling is done, water is in short supply.
Combating lost circulation can be approached in different ways:
- drill ahead with lost circulation;
- drill with a lightweight drilling fluid that will have a static head less than the pore pressure in the formation;
- mix the drilling fluid with fibrous material or particles that will plug the loss apertures in the formation;
- or pause in the drilling, and try to seal the loss zones with some material that can be drilled out as the hole advances.
If an adequate water supply is available, it is practical to drill without returns. If fresh water is not available, produced brine – that normally would be re-injected – can be used for drilling wells within a developed project. Drilling without returns frequently is used when core drilling, where the cuttings are very fine, and where much of the rock comes out of the hole in the form of core. There have been many rotary-drilled holes where intervals of great length have been drilled with complete lost circulation. There are special techniques required to prevent formation collapse and to keep from getting stuck. The highest risk is when only partial returns are obtained, as the low annular velocities above the loss zones may not be adequate to clean the hole. High viscosity sweeps usually are used to reduce this risk. Once total loss is encountered, pumping water at high rates down the annulus, as well as down the drill pipe, will flush the cuttings away from the wellbore, preventing any sticking problems, and providing positive wellbore pressure to hold up weak formations.
Another technology that is useful with lost circulation is dual-tube reverse circulation, but careful consideration must be given to the issues raised about well control.
There are three categories of lightweight fluids – air, foam and aerated fluids from the lowest density to the highest density. Air only can be used where liquid production is minimal or non-existent. Foam will tolerate some water dilution, but not much, while aerated fluids can tolerate a significant amount of dilution.
Aqueous (water-based) foam is attractive because of its simplicity, but it is important to use the proper surfactant that has stable properties at high temperature. Considerable modeling was done in the early development of aqueous foam for geothermal drilling. In addition to numerical models of the foam structure and rheology, a laboratory flow-loop measured pressure, temperature and flow rate at different points, to allow experimental confirmation of a rheological model.
Aerated fluids – liquid with gases injected into it – produce a static head less than or only slightly greater than the pore pressure, and are a common remedy for lost circulation in geothermal drilling. It also reduces the probability of differential sticking. Aerated drilling is used extensively now in many locations, and some claim its use not only avoids problems with lost circulation, but improves the well’s productivity after drilling, although this is a controversial topic in the industry.
Lost-circulation problems generally can be divided into two regimes, differentiated by whether the fracture aperture is smaller or larger than the bit’s nozzle diameter. When severe lost circulation is anticipated, it is usual to run large jets or no jets in the bit, to better accommodate pumping LCM. Clearly, LCM particles that will plug the bit are unacceptable, but for smaller fractures or for matrix permeability, the wellbore theoretically can be sealed by pumping solid or fibrous plugging material mixed with the drilling the fluid. This method is much less effective with larger fractures.
Many substances have been used in the oil-and-gas industry to plug lost-circulation zones, but most of them have been organic or cellulosic materials that cannot withstand geothermal temperatures. This actually is an advantage if the lost-circulation zones are in the productive formations, as the LCM will degrade as the well heats up, minimizing any damage to the productive formations. Lost-circulation zones in oil and gas also tend to be dominated by matrix permeability, rather than the much larger fracture apertures common in geothermal reservoirs. Although traditional organic LCM can be used as long as the circulating temperature prevents degradation, LCM, in general, often has been unsuccessful in geothermal drilling. Several candidate materials that will withstand high temperature have been identified, but they only should be used in the non-productive intervals, because they would permanently plug the productive intervals.
Fractures too large to be plugged by LCM can only be sealed by withdrawing the drill string from the hole and injecting some liquid or viscous material that will enter the fractures, solidify to seal them, and then have its residue removed by resumption of drilling. Conventional lost-circulation treatment practice in geothermal drilling is to position the lower end of an open-end drill pipe near the suspected loss zone, and pump cement downhole. The objective is to emplace enough cement into the loss zone to seal it. However, this does not always occur. There are many issues in getting cement placed into the fractures that are causing the loss zone. Because of its higher density relative to the wellbore fluid, the cement often channels through the wellbore fluid and settles to the bottom of the wellbore (the larger diameters of geothermal wells aggravate this problem).
If the loss zone is not on bottom, the entire wellbore below the loss zone sometimes must be filled with cement before a significant volume of cement flows into the loss zone. Consequently, a large volume of hardened cement often must be drilled to re-open the hole, which wastes time and contaminates the drilling mud with cement fines.
Furthermore, because of the relatively small aperture of many loss-zone fractures, the loss zone may preferentially accept wellbore fluids into the fractures instead of cement, because of the high concentration of solids in cement. This causes dilution of the cement in the loss zone, and loss of integrity of the subsequent cement plug. If there are any cross flows in the wellbore, it will contaminate and dilute the cement, making it impossible to get a cement plug. As a result, multiple cement treatments often are required to plug a single loss zone, with each plug incurring significant time and material costs.
This article is provided through the courtesy of Sandia National Laboratories. It is excerpted from the “Handbook of Best Practices for Geothermal Drilling,” written by John Finger and Doug Blankenship.