Posted on May 4, 2021 by Lisa Ward

Abridged from a presentation by David Cook and Mark Schneider.
View the original poster
Read the full paper

Rose & Associates introduced the Pwell concept at the AAPG 2017 convention in Houston and released the methodology in our Multi-Method Resource Assessment (MMRA) program in early 2018. This method is now available in RoseRA, our current prospect risk analysis software. The Pwell function in RoseRA provides insights into the balance between the chance of success and the potential resources when a well is drilled in a location that is downdiped from the crest of the structure.

Pwell helps users address key issues related to:

Choosing the best downdip location, giving your discoveries a higher probability of finding Estimated Ultimate Recovery (EUR) hydrocarbons exceeding the Minimum Commercial Field Size (MCFS)

Understanding the impact on the chance of geologic and commercial success

Ensuring that, in the event of a dry hole, there will be “no regrets” about potential up-dip volumes tempting a decision-maker to drill a sidetrack or new well up-dip.

The Pwell function in RoseRA requires that an Area distribution be modeled using either the Area-vs-Depth or Area x Net Pay rock volume estimating methods. Users can input the closing contour area at the proposed well location and simulate it to calculate new metrics for the well location including:

The Pwell chances of geologic and commercial

Given a discovery, the range of resources up-dip and downdip from the well location.

Given a dry hole, the range of “attic” resources up-dip from the well location and the up-dip chance of commercial success.

In the following example, the team has proposed a downdip well location at 2,000 acres. The total prospect has a Pg = 50% and a Geologic Mean of 56 mmbo based on an area distribution with P90 of 1,000 acres and P10 of 6,000 acres. The commercial MCFS is 25 mmbo which results in the probability of commercial success being 37.1% and a commercial mean of 70 mmbo. Input the downdip closing contour area of 2,000 acres into RoseRA and run the simulation to observe the results.

Figure 1: Input Area of Closing Contour at Proposed Well Location
Figure 2: Downdip Geologic and Commercial Chance of Success are reported.

The Pwell chance of success decreases the further downdip the well is drilled. In this example, the Pwell geologic chance of success at the 2,000-acre downdip location is 34.0%, much lower than the crestal Pg of 50%. The tradeoff is that the proportion of the commercial downdip resources is high (95.5%), making the Pwell commercial chance (32.5%) only slightly below the Pwell geologic chance. Given success, you are almost guaranteed a commercial accumulation by drilling at the 2,000-acre location.

There is still the possibility of resources being present up-dip if the well location is dry. What is the commercial chance of success for the potential up-dip resources given a dry hole?

Figure 3: Updip Geologic and Commercial Chance of Success are reported.

Pg for the up-dip resources remains the same as the original crestal Pg of 50%. About 41% of the up-dip resource distribution exceeds the MCFS so the Pwell commercial chance is 20.6%. Would that Pwell commercial chance tempt management to drill an expensive sidetrack or a second well to determine whether commercial resources are present? Management would need to know the up-dip resource distribution to make an informed decision.

Use Pwell to investigate the up-dip and downdip geologic and commercial resources as shown in the Pwell log probit below.

Figure 4: The Pwell Log Probit chart shows the Updip (black) and Downdip (blue) resources and Updip Commercial (green) resources.

Updip geologic resources (black) range from P99 of 5 mmbo to a P01 of 66 mmbo with a Mean of 25 mmbo. Downdip geologic resources (blue) range from P99 of 18 mmbo to a P01 of 282 mmbo with a mean of 79 mmbo. The red shading indicates the overlap between the up-dip EUR distribution from P65 to P01 and the downdip EUR distribution from P99 to P45. Resources in excess of about 71 MMBO are not achievable in the up-dip EUR distribution. What is the consequence of the distribution overlap region on decision-making?

With the commercial MCFS of 25 mmbo, the up-dip commercial resources (green) show the up-dip Pmcfs is 41.1% and the up-dip Pc is 20.6%. This commerciality possibility might tempt management to spend additional capital to test for an up-dip commercial accumulation. If this is the case, perhaps the well should be drilled further up-dip to minimize total drilling capital.

Another metric that has been used in the industry to gauge whether or not to spend additional capital is the “No Regrets” resource, which is calculated as the up-dip productive area above the drilled location multiplied by the up-dip mean net pay and the up-dip mean oil yield. Figure 5 shows the “No Regrets” resource is 38 mmbo. Notice that this method provides a deterministic value to aid in decision-making instead of the full up-dip distribution and additional insight that RoseRA provides.

RoseRA provides the ability to model multiple zones, multiple wells, and multiple downdip Pwell areas, shown as vertically stacked zones in a single file. It simulates and reports all entities within the same simulation. Figure 5 shows the up-dip and downdip chances and EURs as increments from the crest to 2,500 acres. Determine which downdip location balances risk, volume, and value for your company.

Figure 5: Sensitivity Analysis of Multiple Well Locations Can Be Done within a Single Simulation

The RoseRA Pwell function reports clear tabular and graphical outputs so that the rationale for drilling the exploration well downdip can be discussed and communicated with team members and management. Pwell helps facilitate the chance of making a commercial discovery while minimizing the need to spend additional exploration capital. Such scenarios might involve drilling a downdip appraisal well to confirm commerciality, or in the event of a dry hole, avoid drilling an up-dip sidetrack or additional well to determine whether there is a “left-behind attic” resource. A dry hole with the potential for up-dip commercial EUR creates a real conundrum. The Pwell analysis with up-dip and downdip distributions can help clarify a better decision.

This methodology can also help users select locations for appraisal wells. The only difference from the exploration example above is that the crestal exploration discovery results in a prospect Pg = 100%, and the Pwell analysis is performed using the updated resources distribution following the discovery well.

Contact Phil Conway and David Cook at Rose & Associates today to get a demo or more information.

The implementation of Pwell in RoseRA is built on the methodology described in the following reference:

 Schneider, F. M., Cook, D. M., Jr. (2017, April 2-5), Drilling a Downdip Location: Effect on Updip and Downdip Resource Estimates and Commercial Chance [Poster session], AAPG 2017 Annual Convention, Houston, TX, United States.