Micronized Cellulose Fibers COMPREHENSIVE OVERVIEW
These products seal pores in permeable low-pressure formations and fractures (natural or induced), to prevent loss of drilling fluid into the formations encountered while drilling and cementing. Eg: Seal, Flake.
The primary function of a lost circulation additive is to plug the zone of loss back in the formation away from the borehole face so that subsequent operations will not result in additional drilling fluids loss.
In oil or gas well drilling, lost circulation occurs when drilling fluid, known commonly as "mud", flows into one or more geological formations instead of returning up the annulus. Lost circulation can be a serious problem during the drilling of an oil well or gas well.
Consequences
The consequences of lost circulation can be as little as the loss of a few dollars of drilling fluid or as disastrous as a blowout and loss of life, so close monitoring of tanks, pits, and flow from the well, to quickly assess and control lost circulation, is taught and practiced. If the amount of fluid in the wellbore drops due to lost circulation (or any other reason), hydrostatic pressure is reduced, which can allow a gas or fluid which is under a higher pressure than the reduced hydrostatic pressure to flow into the wellbore.
Another consequence of lost circulation is dry drilling. Dry drilling occurs when fluid is completely lost from the well bore without actual drilling coming to a stop. The effects of dry drilling range from as minor as destroying a bit to as serious as major damage to the wellbore requiring a new well to be drilled. Dry drilling can also cause severe damage to the drill string, including snapping the pipe, and the drilling rig itself.
Maximize fluid retention and wellbore stability using our specialized fluid loss control additives. These solutions not only enhance drilling efficiency but also contribute to cost-effective operations.
Control
Although preferred, stopping lost circulation completely is not always possible or required. Controlled losses allow drilling to continue while keeping the wellbore full, preventing an influx of gas or fluid into the wellbore, known as a" kick", which can lead to a blowout.
A number of options are available when lost circulation occurs, depending on the severity.
Minor losses may be controlled by increasing the viscosity of the fluid with bentonite and/or polymers, or with the addition of other additives, which typically includes sawdust.
Severe losses will require increasing the viscosity of the fluid with bentonite and/or polymers and the addition of other additives, which typically includes sawdust. Total losses can be regained through conventional use of increased viscosity and additives, or through use of unconventional methods such as pumping of golf balls, tree branches, rags, additive sacks, and other items in conjunction with, or followed by, a high viscosity fluid. If total losses occur and circulation cannot be regained, several options are available, depending on the operational requirements and depth being drilled in relation to desired production geological zones.
Continuing drilling while pumping drilling fluid is one option, though continued drilling while pumping water is less costly and more often used. Sometimes the cuttings from continued drilling will aid in reducing leaks or stop losses altogether. A third option is to cement the zone where the losses occurred, and to drill through the cement and continue drilling the well. This third option is very often the most cost effective if severe losses occur, as lost circulation sometimes cannot be controlled with other methods.
Additives
The most common additive used to control or stop lost circulation is bentonite, which will seal small holes or fractures. Bentonite, in higher concentrations, increases viscosity, and therefore slows the fluid flow into the surrounding rock. Although bentonite is the most common additive used, it sometimes is not used at all, depending on the fluid being used and the depth of drilling in relation to desired production zones.
Polymers are also sometimes used to increase the viscosity. Though these are more costly, they are more compatible with several types of fluid systems.
There is a great variety of additives which physically plug or seal the losses, including sawdust, flaked cellophane, and crushed or ground gypsum.
Other common and cheaper additives are shredded newspaper and cotton seed hulls. Cotton seed hulls are less preferred as they may cause wear to pump swabs and springs.
Both of these are generally only used when either fresh or brine water is being used for the drilling fluid.
When drilling in salt formations, brine water is typically used, as it does not as readily dissolve salt, preventing the formation of washouts. Washouts not only contribute to loss of circulation, but can jeopardize the integrity of the wellbore itself.
Additive Considerations
Several factors are considered in what additives are used:
- Hole size currently being drilled.
- Drilling fluid in use. The additives must be compatible.
- Depth of the well in regards to geological stability.
- Depth of well in relation to the desired production zones. Plugging a production zone is not a desired outcome.
- Drill bit nozzles sizes. If the additive(s) will not go through the drill bit, they cannot be used.
- Other drill string mechanical equipment such as a mud motor or MWD tools. If the additive(s) will not go through the drill string, they cannot be used.
Formation Kick
The downhole fluid pressures are controlled in modern wells through the balancing of the hydrostatic pressure provided by the mud used. Should the balance of the drilling mud pressure be incorrect then formation fluids (oil, natural gas and/or water) begin to flow into the wellbore and up the annulus (the space between the outside of the drill string and the walls of the open hole or the inside of the last casing string set), and/or inside the drill pipe. This is commonly called a kick.
If the well is not shut in (common term for the closing of the blow-out preventer valves), a kick can quickly escalate into a blowout when the formation fluids reach the surface, especially when the influx contains gas that expands rapidly as it flows up the wellbore, further decreasing the effective weight of the fluid. In other PETRAleum engineering words, the formation pore pressure gradient exceeds the mud pressure gradient, even in some cases when the Equivalent Circulating Density ECD is imposed with the mud pumps on the rig. Additional mechanical barriers such as blowout preventers (BOPs) can be closed to isolate the well while the hydrostatic balance is regained through circulation of fluids in the well.
Early warning signs of a well kick are:
- Sudden change in drilling rate;
- Change in surface fluid rate;
- Change in pump pressure;
- Reduction in drillpipe weight;
- Surface mud cut by gas, oil or water;
- Connection gases, high background gas units, and high bottoms up gas units in the mudlogging unit.
The primary means of detecting a kick is a relative change in the circulation rate back up to the surface into the mud pits. The drilling crew or mud engineer keeps track of the level in the mud pits and/or closely monitors the rate of mud returns versus the rate that is being pumped down the drill pipe. Upon encountering a zone of higher pressure than is being exerted by the hydrostatic head of the drilling mud at the bit, an increase in mud returns would be noticed as the formation fluid influx pushes the drilling mud toward the surface at a higher rate. Conversely, if the rate of returns is slower than expected, it means that a certain amount of the mud is being lost to a thief zone somewhere below the last casing shoe. This does not necessarily result in a kick (and may never become one); however, a drop in the mud level might allow influx of formation fluids from other zones if the hydrostatic head at depth is reduced to less than that of a full column of mud.
Well Control
The first response to detecting a kick would be to isolate the wellbore from the surface by activating the blow-out preventers and closing in the well. Then the drilling crew would attempt to circulate in a heavier kill fluid to increase the hydrostatic pressure (sometimes with the assistance of a well control company). In the process, the influx fluids will be slowly circulated out in a controlled manner, taking care not to allow any gas to accelerate up the wellbore too quickly by controlling casing pressure with chokes on a predetermined schedule.
This effect will be minor if the influx fluid is mainly salt water. And with an oil-based drilling fluid it can be masked in the early stages of controlling a kick because gas influx may dissolve into the oil under pressure at depth, only to come out of solution and expand rather rapidly as the influx nears the surface. Once all the contaminant has been circulated out, the casing pressure should have reached zero.