Common Geosteering Mistakes and How to Avoid them
Over the years, a number of errors in the Geosteering process have been identified and dissected. We will briefly describe some common mistakes.
Depend Too Much On Pre-Planning
Pre-planning is an important way to cover as many contingencies as possible, but it is very important to recognize that the pre-drill interpretation will certainly change once the well kicks off. Coming to depend too much on the pre-drill plan will often blind the Geosteering Team to changes that actually deserve immediate reaction.
It is best to make the pre-drill plan as simple as possible, and then let the observed data dictate changes that should be made to the well trajectory. If the geology matches the model exactly, then crucial data is either not being recorded or is probably being ignored.
Simplistic models can generally change quite easily whenever observed data is introduced. Conversely, complex, pre-planned wellbore geometry is rarely achieved. It is important to remember that few plans survive contact with the formation. The pre-drill plan should be used to help make decisions more quickly, but not to make decisions unnecessary.
Too Much Faith in the Technology
Determining how a BHA will drill is not an exact science. Build rates are affected by many factors which are outside the Directional Driller’s control, so it is often difficult to determine exactly how the BHA will perform. When steering to a new target in the lateral section, be certain to allow for a margin for error.
LWD tool performance tends to be very good, but only to the extent that their data can be interpreted by a human being. One can have the best tools available on the job, but it is the people that will make the difference as to whether a well will be in zone. NEVER EXPECT TECHNOLOGY TO REPLACE TALENTED PEOPLE.
A law common to the Oil Field is that contractors never admit their tool is not working unless it is in more than two pieces when it is brought back to the surface. Be mindful that these tools do sometimes provide misleading data. This again comes back to experienced people. Someone that has interpreted quite a bit of data can generally recognize when a tool is giving erroneous data.
Over-Interpretation of Data
It is quite possible to over-interpret data. A good rule of thumb is that it will not be necessary to collate and review the data any more often than one would be willing to make a target change.
The danger in over-interpretation lies in the tendency to miss the overall pattern of the data, thus causing a greater tendency to force the data to match a preconceived model. This is especially true when using a correlation method to interpret LWD data. Over-interpretation thus has the potential to cause too many steering decisions to be made, and in so doing, may jeopardize the hole. If a new target is stated every 100 feet, the Directional Driller will become frustrated and may even begin to be less cooperative.
Over Correction When Drilling Out of Zone
More holes are lost because of over-steering than for any other reason. If a transition from one target to the next creates a bad dogleg, it will adversely affect the distance a well can be drilled. With two or more bad doglegs in close succession (especially if they are in opposite directions), the risk of sticking pipe in the hole rises dramatically.
All target transitions should be smooth and gradual, even if it means that the well will spend a little time out of zone.
Geosteering by Compromise
Never split the difference between two interpretations.
Either wait until the two interpretations are resolved, or make the decision based upon which interpretation makes the best geologic sense. The Team Leader should ultimately decide which interpretation agrees the most with all other geonavigation evidence.
Failing to Use the Right Combination of Tools, Evaluation Methods, and Personnel
Many operators try to save money by not using the proper evaluation tools for the job. Remember that a cheaply drilled well which is not in zone is actually very expensive! Adding cost to a well for the right reason will pay high dividends once the well starts producing.
Much effort, planning, and money are involved in horizontal drilling projects. Beginning long before the first well is drilled, the horizontal well springs from a knowledge of the reservoir and its stratigraphic and structural characteristics. A plan is built for the well based upon that knowledge of the reservoir. The team must then prioritize the information needed for the project and choose the right tools to accomplish the goals prescribed in the optimal plan.
Selecting the best method to interpret the data, and finding experienced personnel to make the steering decisions will add value to any horizontal project. Once everything is in place and all contingency plans are made, the horizontal well can be drilled with confidence that the objectives will be met.
