Drilling waste minimization or reuse of resources that can become waste are key strategies in waste avoidance and a sound waste management plan. Two general approaches to waste minimization have developed. They can be called total fluid management (TFM) and environmental impact reduction (EIR).
Total Fluid Management
One of the largest sources of drilling waste for onshore operations is location water. This happens to be the source that can be reduced most. Most wastewater originates from drilling-fluid usage, storm water, rig wash water, or cooling water. The volume of location water requiring handling and disposal could be as much as 30 times the hole volume.
Table 1. shows data collected from a project in Louisiana for wells drilled during 1995 and 1996. The first well established the baseline. Water reduction schemes began on the following wells. Some of the wells were concurrent because two rigs were used for the project.
Table 1. Waste Handled on Similar Wells
|Well||Days||HV(bbl)||Waste Handal(bbl)||Ratio Waste:HV|
HV = Hole volume
While the hole volume remained fairly constant throughout the project, waste volume was reduced to about one-half the original amount. The later wells (#5 and #7) had waste volumes consistent with multiples of 15 times hole volume. One well (#6) achieved a waste volume multiple of 8 times hole volume. This well began using oil-based fluid earlier than the rest, which reduced total waste volume. However, this was an economic failure due to low penetration rate and higher cost of drilling.
The approaches taken to reduce wastewater generation were based on reuse of as much water as possible. They included the following techniques:
- Single-pass systems, such as cooling water, brake water, and seal water, were eliminated. These should be contained by enclosed systems. Recycling these fluids is inexpensive and can save a large amount of fluid.
- Storm water was reused. Storm water can be reused for fluid makeup water, although the drilling personnel may not like it much. It can also be used for rig wash water. Rig wash water (which falls into the same ditches as the storm water) should be reused until it is too dirty to be used as wash water. It is surprising how many times wash water can be used effectively.
- The dirtiest water (such as drilling-fluid waste) was used for slide wash water. Desanders and desilters generate copious amounts of drilling-fluid waste (usually calculated at two or three parts liquid to one part solid), yet still require washing to the disposal pit. Shaker slides and centrifuge slides almost always require wash water. Slide wash water does not need to be clean, and the introduction of any clean water into the waste solids and fluid chemicals is an unnecessary addition of water that becomes difficult to separate during disposal.
- Liquid waste was not generated needlessly. The use of rig vacuums rather than washing is increasing precisely because of the expense involved with disposal of waste liquids. Pistol-grip shutoff valves on hoses are a great idea. When the floor hand is called for a connection, the hose that is thrown down will shut off automatically rather than run the whole time during connections. High-pressure/low-volume washers are a favorite with rig crews, because they clean better with less effort. They also save liquid waste volume. Vacuums and washers are usually a breakeven cost unless the disposal cost is high, but pistol grips always pay off.
- Wastes that were to be handled in different ways were separated. For instance, do not combine oil-based wastes with water-based wastes, unless they will be handled together. In this project, all liquid from the reserve pit was injected, so all liquid went to the reserve pit.
Preplanning was an integral part of the Louisiana project. Thinking about the operation and developing a plan to handle the waste streams always pays off. If you are going to dump the sand trap, then where will that waste stream go (especially if your pits are aboveground)? How will you handle the low-contaminant but large volume associated with surface hole and keep it separated from the oil-based fluid or highly treated fluid later? Once the location is built, it is usually too late to consider these things.
As can be seen, the TFM technique can be used to reduce waste
volume or amount. However, the reduction in waste is mostly by
reducing the amount of water in the waste. While this may not seem
important to some, it is nonetheless saving a valuable resource and will
prove cost-effective as disposal costs continue to escalate.
Environmental Impact Reduction
Another form of minimization strategy is to evaluate the environmental
impact of the project and attempt to reduce it. In the EIR method, all
fluids are evaluated for their chemical components. Certain environmental
data are collected on each of the chemicals. The data might
include parameters of
- biodegradation potential
- heavy metal concentrations
A review of the chemicals to be used would be made, and those chemicals
with the least environmental impact would be selected.
A simple example of this is prequalifying a drilling-fluid system. In the
prequalification, every chemical to be used is examined for the desired
environmental characteristics and approved for use. In addition to each
chemical individually, the entire system would be approved. Only
approved chemicals, and only at the maximum approved concentration,
would be allowed. This is, of course, a very complex system. Many fluid
programs contain contingency chemicals that are used under only certain
circumstances for a small portion of the hole.