The inadvertent return of drilling fluid is common in horizontal directional drilling (HDD) projects, and though not harmful to the environment, inadvertent returns are undesirable for a number of reasons. The key to preventing inadvertent returns is to keep the downhole pressure as low as possible.
What is an Inadvertent Return?
An inadvertent return or IR is the unintentional return of drilling fluid to the surface by paths other than the mud recycling system. During a drilling operation, varying geology along the project route can cause drilling fluid to find its way to the surface via natural crevices, such as cracks and fissures. Thomas Jamieson, VP Engineered Solutions, TriFlo, says that it is impossible to anticipate an IR until it comes flowing in, but one can be prepared for it.
IR typically occurs close to the entry and exit points where the soil cover is thin and soil strength is low. The drilling fluid can flow into underground formations and come out at the surface such as on roads, railroads, wetlands, and utility trenches. An IR can release large amounts of drilling mud in a short time if the high-pressure drilling fluid pumps are not stopped. (Read also: The Importance of Fluids Loss Control in Drilling Operations.)
Downhole Pressure and Inadvertent Returns
The risk of inadvertent returns is greatly increased when the drilling mud pressure is higher than the confining strength of the formation. During the design phase for an HDD project, it is necessary to determine an appropriate depth that will confine the drilling fluid within the borehole. Inadequate depth or a low strength formation will allow the pressurized drilling fluid to escape to the surface when the pressure exceeds the confining strength of the soil.
Downhole pressure plays a major role in causing an inadvertent return. According to Tyson Smith, Technical Sales Engineer, Wyo-Ben, the following factors increase the chances of an IR.
A Dirty Borehole:
Drilling fluids follow the path of least resistance, such as through an open, clean borehole. In a borehole that is not clean because the cuttings are not removed properly, resistance is offered to the flow of the drilling fluid. If a fracture, fissure, or unconsolidated soil is encountered, the drilling fluid will flow through it rather than the borehole, causing an IR.
Heavy Drilling Fluid:
Drilling fluid that is heavy and laden with cuttings will take more pressure to move. Enough fluid should be pumped and the mud recycler should be given enough time to perform its job of removing the drill cuttings. If a recycled drilling fluid weighs more than 10 lbs. /gallon, it should be diluted or discarded.
Low or High Gel Strengths:
A drilling fluid should have the right gel strength to suspend the cuttings and support the bore during a stop. Low gel strength will make the cuttings settle while high gel strength will make the mud thicken during a stop. This will require high pump pressure to move the drilling fluid again.
Soil/Rock Formation and Inadvertent Returns
HDD projects encounter sub-surface soil and rock strata that may sometimes change drastically. The drilling operation may begin with clay and then suddenly encounter sand or rock. If the right systems are not in place, it can cause the drilling fluid to change characteristics, increase downhole pressure, not remove cuttings properly, and cause an IR by exiting from any path of least resistance in the borehole such as rock substrates, fissures, and cracks.
To prepare for such a scenario, it is necessary to conduct a thorough geotechnical investigation. This obtains sufficient information that can be relied on to design and install permanent works. Based on the report, the appropriate route for the HDD project, the right mud mix, selection of the solids control system, and right size of shaker screens can be determined.
Knowledge of ground and groundwater conditions reduces the risk of inadvertent releases and allows the contractor to be prepared with an inadvertent release plan to address site-specific conditions. (Read also: HDD in Tough Conditions: Between a Rock and a Hard Place.)
Key Methods for Controlling/Reducing Downhole Pressure
Monitoring Drilling Operations
An annular pressure tool installed on the front of the bottom hole assembly (BHA) with a pressure port at the side of the tool allows the downhole steering probe to sense annular pressure in the hole. Greg Bosch, Business Development, Mears Group, Inc. says that pressure-monitoring tools are a great way to predict where a potential IR can take place. Continuous monitoring of annular pressure helps in determining variations from the estimated downhole pressure. Higher pressure indicates a restriction in the hole that the contractor can immediately rectify before IR takes place.
Right Mix of Drilling Fluid
The right mix of drilling fluid is key to preventing inadvertent returns. The drilling fluid should have the right gel strength, plastic viscosity, and yield point, for which appropriate additives should be used. A simple mud recipe will have water as the base, bentonite clay, and polyanionic cellulose (PAC) polymers such as FloMaster and Uni-Drill.
Using the Right Additives
Additives used in drilling fluid are non-toxic and do not harm the environment. Some of the additives are also used in products such as ice cream, cosmetics, cholesterol medications, and toothpaste.
Bentonite such as Tru-Bore by Wyo-Ben provides excellent viscosity and gel strength to carry cuttings and support the bore in addition to providing fluid loss control properties
Xanthan gum is used for providing gel strength and is a biopolymer. Borzan D by Wyo-Ben is a highly concentrated xanthan gum used in vertical and horizontal drilling operations where higher gel strengths and low downhole pressures are required. It can also be used as a standalone drilling fluid or as an additive to enhance bentonite-based drilling fluids, enhance gel strengths, control filtrate, and inhibit clays.
PHPA provides excellent viscosity and clay inhibition properties but does not provide gel strengths
Drilling lubricants such as Wyo-Lube RR by Wyo-Ben reduces torque and drag, increases penetration rates, and extends drill bit and rod life. It allows the drilling rig to be used at maximum potential and provides the operator increased control level.
- Good Solids Control Equipment
Removing cuttings from the borehole is very essential to preventing inadvertent returns. Solids control aims at removing as much of the large cuttings out of the drilling mud when it is first pumped out of the borehole. The solids removal treating equipment is designed and installed and sized to process at least 100 to 125% of the mud-circulation rate.
A well-trained operator will be able to detect changes in the mud rheology, loss of circulation pressure, and loss of mud volume in the mud pit. Correcting the problem as soon as it is detected can help prevent an IR. (Read also: Trenchless Frac-Out: How Do You Prevent It?)
Dealing with Inadvertent Return
When inadvertent returns happen, they have to be tackled quickly. The initial response to a potential IR can be as follows:
Immediately reduce drilling fluid circulating pressure, continue drill string rotation, and keep advancing the drill head to stop or reduce the IR release rate while avoiding the creation of a downhole cavity.
If there is no continued release of fluid at the IR release point, the operator can continue rotating the drill string and circulate fluid at lower pressure to keep the borehole open.
Suspend advancement of the drill if there is a failure to remove cuttings.
Use lost circulation material like Plugz-It Max that are designed to mix and pump with a drilling fluid into cobble, gravel, or fractured zones to restore mud circulation.
Since most IRs happen near the entry and exit points, drive casings at both sides.
Inadvertent returns can cause project delays and result in costs you hadn't anticipated. Having a plan in place to deal with them if they do occur is important, but with the right planning, knowledge and resources, you can be successful at preventing inadvertent returns all together.