An integrated real-time geopressure, earth stress and wellbore stability analysis for drilling operations can save wells.

Many high yield, high expense, exploration and development operations are hampered by formation fluid influx (kick), drilling fluid loss (hydraulic fracture) and wellbore instability due to shear failure. These events are estimated to cost the industry almost US $8 billion every year, and this number has not decreased despite advances in pre-drill modeling. Indeed, a recent operator study cited 44% of their non-productive time (NPT) was associated with improper management of annular pressures leading to geopressure and wellbore instability related delays in well construction.

These unscheduled events result when annular pressures stray from inside the safe operating envelope defined by shear failure pressure, pore pressure and fracture pressure. The annular pressures must be constrained inside this "safe" envelope if these time consuming events are to be minimized or avoided.
The integration of pre-drill geopressure and geomechanics analyses with real-time analysis has been a consistently effective strategy for mitigating pre-drill uncertainties and subsequently improving well construction and evaluation efficiency. This article documents a proven process to reduce uncertainty and mitigate drilling risk associated with formation fluid influx (kick), drilling fluid loss (hydraulic fracture) and wellbore instability (shear failure).

Intractable uncertainties in pre-drill geopressure and wellbore stability prognoses require that the models are considered "dynamic" so that they are continually updated and calibrated, in real-time as new data is acquired. Below is a safe operating envelope graphic from a Deepwater Gulf of Mexico project. Notice that the pre-drill operating envelope for fluid was prognosed to be greater than 2 pounds per gallon (ppg) at the narrowest point. In fact, data acquired in real-time proved that the envelope was less than .6 ppg.

In this case, the operator was able to navigate the narrow window using an expert, real-time analysis. Unfortunately, many other projects have not benefited from the implementation of such a process/analysis.

The utility of a commercial software package in which geopressure, earth stress and wellbore stability analyses are fully integrated allows data and expertise to move from well planning through well construction. This process ensures consistency and begins with the pre-drill prognoses in the well planning stages. Post-spud, real-time at the bit analysis and updated ahead of the bit forecasts are created for geopressure, earth stress and wellbore stability analyses. These three analyses create an envelope of safe operating pressures, which should constrain the annular pressures to prevent formation fluid influx, drilling fluid loss, or wellbore instability. Wellbore stability analysis is critically coupled to geopressure and earth stress analysis. Therefore, it is critical that wellbore stability modeling be integrated with geopressure and earth stress modeling as described in this work. In fact, it is this optimization of workflow that adds the principal value to the operator by allowing the actionable information to get to the right people, at the right time, during well construction.

Drilling observations such as torque and hole-fill, gas, cuttings and cavings, analysis are coupled with electric logging data from wireline or logging while drilling (LWD) to create a timely and comprehensive analysis for drilling. In addition to creating an envelope for an annular pressure management strategy, the analyses can be used to anticipate and diagnose wellbore failures ahead of the bit. Furthermore, the safe operating envelopes created at and ahead of the bit are used to provide critical information necessary to optimize the casing and drilling fluid programs and reduce the probability of unscheduled events. The aforementioned technology and process has been used successfully by several operators on numerous drilling campaigns around the world.

First, let us examine a case that could have benefited from a real-time analysis of the safe operating envelope. Operations with construction budgets of less than $7 million typically do not consider the cost-benefit relationship suitable for comprehensive NPT reduction strategies; however, as this case will show, that is a costly assumption. The following is a Gulf of Mexico well. The well program was a simple slant with a 10,000 ft (3,050 m) Kick-off point and 2,500 ft (762 m) in which to build to 50? of inclination. The target was at about 15,000 ft (4,575 m) measured depth (MD).
The pre-drill pore pressure prognosis was based solely upon offset mud weights, and appeared benign. No pre-drill wellbore stability analysis was performed by the operator. Maximum required mud weight was expected to be less than 10.9 ppg. Drilling proceeded for approximately 5,000 ft to 10,000 ft (1,525 m to 3,050 m) MD, at which time inclination was built per the drilling program. Almost immediately following the kick-off, tight hole and hole fill were reported. The problems persisted and increased in frequency and magnitude through the build section. Stuck pipe was encountered and freed; however, drilling conditions were difficult. Hole conditions finally deteriorated to the point where a mud weight increase was ordered. Six tenths were added to achieve an 11.5 ppg at 13,820 ft (4,215 m) MD; however the problems persisted and so did drilling: the mud weight was again raised at 14,130 ft (4,309 m) MD, with similar, poor results. Finally, at approximately 14,250 ft (4,346 m) MD, a well flow was observed, and the entire openhole was lost during the kill operations. This cost the operator almost $5 million and several weeks of time.
What happened to this project? Figure 1 tells the story.

While the mud weights selected during the pre-drill were adequate to prevent fluid influx to 13,750 ft (4,193 m) MD, they were insufficient to prevent wellbore collapse. The collapse pressures exceeding the pore pressures resulted from the wellbore geometry. As inclination was built, stress anisotropy built around the well bore as the difference between the horizontal and vertical stress components mounted. In the Gulf of Mexico, the horizontal stresses are frequently very similar in magnitude, and shear failure in vertical and near vertical wells is generally not problematic. Unfortunately, as inclination is built, the differences in the vertical and horizontal components do create shear stresses and these must be managed with additional annular fluid pressure (mud weight).

The wellbore instability was certainly problematic, but not catastrophic. The final blow to this operation occurred in the form of a formation fluid influx (kick). The kick was not detectable using gas or drilling indicators. However, a simple resistivity analysis performed by a competent analyst with state of the art technology, accurately prognosed the location and magnitude of the event with little difficulty. While this was done in hindsight, there is no reason to believe that a similar analysis could not have been done in real-time, with the identical information.

What can be learned from this example? All operations must properly consider the implications of improperly managing the safe operating envelope during well construction. The proper management of this envelope begins with the pre-drill quantification of pore pressures, fracture pressures and shear failure pressures. Subsequently, this analysis must be updated as new information becomes available at the well site. In this case, the survey information coupled with LWD resistivity and gamma ray was sufficient to predict the events that eventually caused the loss of the well bore. While more measurements are always useful, and better models are always available, operators should know cost effective solutions are available to match the economics of their drilling program that will impact their success rate. As previously mentioned, the key is getting the actionable information into the decision making process at the right time.

To maximize the distillation of data into actionable information, a process needs to be implemented that benefits from the synergy of expert experience, technology, best modeling practices and team organization. The use of a field deployable geopressure analyst to build, monitor and communicate the subsurface safe operating envelope is the most effective way to reduce unscheduled events and non-productive time related to formation fluid influx, shear failure and drilling fluid loss. This analyst does not work outside or beneath the existing teams. Instead the analyst must become a central component who is accountable for getting the best possible information into the decision-making process at the right time.
This system has been in use for several years with over a hundred projects, perhaps hundreds of projects, benefiting from its use.

The following is an example of proper management of the safe operating envelope.

Non-productive time associated with pore pressure, fracture pressure and collapse pressure is a serious problem. It is proposed that these problems cost the industry billions of dollars annually and may represent 40%-50% of an operator's total non-productive time related to well construction. Advances in modeling and monitoring have been made over the last decade. However, there have not been concomitant reductions in associated drilling problems. The use of a comprehensive process to address these costly problems is the solution. While this system includes advanced technology and modeling, the principal gain is derived from the correct insertion of actionable information into the decision-making process. This is primarily achieved through the use of an accountable analyst responsible for building, maintaining and communicating the safe operating envelope to the drilling team's decision makers. This allows the team to respond proactively, instead of reactively and mitigate problems before the delay in well construction.