One of the Reasons of Why Horizontal Wells Underperform, Part II: 3 Facts Revealed by Inspection
Updated: Apr 23
Wellbore Inspection Revealed a Stationary Bed of Sand in the Lateral
Abrado Wellbore Services completed a job in the Gulf of Mexico (GOM) with their memory camera. The video they posted (shown above) was used by their client to make a decision regarding "resuming production" at a gas-lift well, i.e., the well had been producing before the wellbore inspection was carried out. If this is true, the following facts are demonstrated in this video (its second part, MD > 12,000 ft):
A stationary bed of sand was formed in the lateral wellbore during the production period.
Sand holdup (amount of deposited sand) is greater in zones located near the flow ports or at locations where the internal diameter of the liner is increased.
Sand holdup is greater in the sliding sleeve nearest to the well toe.
The oil flow rate in unconventional wells is one order of magnitude smaller than that in GOM wells. Also, the amount of solids (proppant) entering into the lateral of a hydraulically fractured well is greater than that in conventional horizontal wells. So, if sand deposition occurs in an offshore horizontal well, as shown in the video, a stationary bed of proppant is definitely formed in one or multiple stages of a hydraulically fractured well, as it is demonstrated in my previous articles.
Scale-model tests conducted for the development of our related technology solution also confirm the results listed above. For these reasons, the risk of impaired production caused by the bed formation in the unconventional wells is high and should not be underestimated.
A Way Forward
Shale oil and gas producers are increasing the length of the lateral and proppant intensity, as well as are experimenting with different completion techniques to increase EUR. Some of them are trying to improve well economics by reducing the choke opening size at the wellhead.
The first approach (long laterals) is a resource intensive one and does not eliminate the risk of impaired well productivity. Also, these methods have some limitations. In the article Changes in shale well design: Reaching the limits?, Emilie Gubian, Principal Economist, Performance Evaluator at IHS Markit says
For the system as a whole, we believe that running room remains and will continue to raise well productivity by creating longer, more intense wells. However, over time, we believe those gains will peter out. Using a baseball analogy, we would submit that
Lateral length extension is likely in the 7th inning, primarily because leasehold configuration is constraining many operators in the Bakken and Permian.
Proppant intensity strikes as being in the 4th inning.
Furthermore, if you look at your historical production data, you will find that EUR of a short-lateral well may be much higher than that of a long-lateral well, it is a commonplace. Reservoir engineers have proposed several methods for predicting the inflow performance relationship (IPR) of horizontal wells. According to them, IPR (linearly) increases with the lateral length. The skin factor caused the proppant bed formation and the risk of plugging of the lateral are not considered in these methods. It is a well-known fact that IPR calculations in many cases overpredict the well production rate, at least 12 months after initial production.
Undoubtedly, there are many contributing factors to this discrepancy, however proppant transport in the lateral is one of the key aspects of the problem. If you look at the figure at the top of this page, you will find that oil and water are essentially stagnant in many stages near the flow ports, e.g. in Stage 5,6, and 7. It is an Ultra-High Definition (UHD) 3-phase gas-oil-water flow simulation that (intentionally) does not consider proppant flowback, as the above mentioned IPR equations. Proppant cannot be transported along the lateral in stagnant or near-stagnant fluids, where sand particles settle down forming a stationary bed over time.
This simulation is included in the present article just to illustrate the cause of sand deposition. Actual distribution of phases along the lateral predicted by the UHD well model, including proppant, is presented in the second article in section Referencies. It adequately reproduces flow behavior observed in the video of wellbore inspection.
The second approach (choking) may initially work for gas wells or wells producing a high-GOR crude oil. But, over time, once water accumulated in the production liner becomes stagnant sand deposition will occur in many stages too.
To summarize, a next step in production technology development for unconventional wells needs to be done to increase EUR and retain/attract more investments to the unconventional resources industry. Applying existing technologies, even with some modifications (more sand, longer laterals, more stages, etc.), "over and over again" will not generate substantial improvements in returns on capital expenditures. These widely known words from the world's most famous scientist perfectly describe the current situation. The Abrado´s video and the line chart in the figure above confirm that a technical solution, based on an in-depth understanding of how proppant and water are transported along the lateral over the lifetime of the well, is a way forward. Returning to the baseball analogy, here we are about to start the 1-st inning.
If you find this article helpful, you might be interested in learning more about flow assurance in unconventional wells by visiting this web site: www.mpecorp.com/flow-assurance-services-for-unconventional-wells. You can reach me at: email@example.com