China Workshop Examines Fractured Carbonate Reservoirs
An SPE Applied Technology Workshop (ATW) on “The Characterization of Effective and Efficient Development of Fractured Carbonate Reservoirs” was held 15-18 June 2014 in Nanjing, China. Participants reviewed recent advances in exploration and production and the technical challenges presented by these reservoirs.
A variety of carbonate reservoirs exist worldwide, mainly paleokarst reservoirs. Because of various scales, poor continuity, strong heterogeneity, accumulation in vugs and fractures, and complex flow behaviors, the efficient development of fractured carbonate reservoirs is quite challenging.
Li Yang, cochairperson of the workshop, opened the session and ATW Chairman Wang Zhigang delivered one of two keynote speeches. He focused on the challenges and current activities of a key carbonate reservoir, Tahe, that he said represented the theme of the ATW. Usman Ahmed, also a workshop cochairperson, introduced the second keynote speaker, Christine Ehlig-Economides. She recalled an ATW a decade ago that also had focused on the Tahe reservoir. She set the stage for a discussion on how much the industry has advanced and the challenges that lie ahead.
In his keynote speech, Wang of Sinopec described the technical challenges and techniques used at Sinopec’s Tahe oil field in western China. The Tarim basin is the company’s largest carbonate play, with proven reserves of 1.3 billion tons and depths of 5300 m to 6300 m. The oil and gas accumulated in pores, vugs, and fractures of severe heterogeneity and depth make it difficult to predict and describe reservoir characteristics, he said.
Carbonate reservoir characterization and evaluation techniques have been developed that have improved the predictability of the carbonate reservoir and its distribution and have increased drilling success, he said. In the Ordovician carbonate reservoir of Tahe, a series of techniques such as paleokarst landform categorization, paleokarst cycle division, hydrocarbon accumulation period analysis, logging for reservoir identification, and evaluation have been implemented. In addition, massive acid fracturing (high pumping pressure, high injection rate, large scale) has become a major technique for development at the field.
In sum, the technical challenges are:
- Ultradeep, highly heterogeneous reservoirs with low-resolution seismic data make it difficult for reservoir prediction.
- The form of fractures and vugs and oil and gas accumulation in such reservoirs make reservoir modeling difficult.
- A variety of reservoir fluid systems exist, which lead to a complex oil-water distribution. Current simulation techniques need to be further improved.
- Severe channeling and breakthrough were detected after waterflooding was applied. Further research is needed to improve waterflooding, residual oil characterization, and water-control techniques.
- Drilling activities are extremely challenging and costly. Drilling technology needs to be further developed or modified.
Challenges As We Know Today
The first workshop discussion, titled “Oil and Gas Production From Sinopec Carbonate Reservoirs in China: Current Status and Challenges,” was led by Duan Taizhong of Sinopec. Duan described the Tahe and Futai oil fields and the Puguang and Yuanba gas fields. He described the Tahe field geology as multiple superimposed karstification systems related to several unconformities with flow types that include free, pipeline, interstitial, Darcy or non-Darcy, and linear or nonlinear, and dominated by fracture cavern behavior. Frequently, oil production stops with sudden water production, he said.
Three reservoir types in the Futai field are fracture-vug, fracture only, and fracture matrix. Water injection has not been an efficient way to supplement insufficient internal reservoir energy to support production, he said. Production in the Puguang gas field has not yet shown evidence of fractures seen in seismic and log data, but is consistent with carbonate facies clearly identified in seismic data. Production from the Yuanba field is in its initial stages.
The second discussion, led by Heber Cinco-Ley, was about the challenges he has faced in working with carbonate fields in Mexico, including Cantarell, Jujo-Teconimoacan, Abkatun-Pol-Chuk, Homol, and Kuil. In the Cantarell field, very good vertical hydraulic communication in the reservoir resulted in quick formation of a secondary gas cap, and nitrogen injection was used to maintain reservoir pressure. Conductive faults act like constant pressure boundaries in pressure transient data and result in water entry into the oil zone. The reservoir is highly connected by conductive faults and fractures, and small caves exist that do not dominate well production behavior. The Abkatun-Pol-Chuk, Homol, and Kuil reservoirs are connected to the same aquifer, and activity in any one of the reservoirs may affect the others; there is evidence that low permeability areas in the reservoirs are produced via connected higher permeability reservoir areas.
Understanding the Subsurface
The next discussion focused on seismic attributes that can be used to detect and characterize fractures and karst features, but need to be validated with static and dynamic data. Among the conclusions are:
- An understanding of in-situ stresses is important in designing well trajectory and direction.
- Nuclear magnetic resonance techniques can be used to understand pore and fracture types.
- Temperature effect on relative permeability is not significant for unconsolidated carbonates.
Characterization of fractured cavity formation in the Tahe oil field presents two problems:
- Fine recognition of internal structure of fractured-cavity formation
- Quantitative characterization of fractured-cavity formation
The question was posed: Why is the difference between the apparent volume of fractured-cavity formation predicated by seismic and the true volume obvious? Participants recommended correcting the model by use of all kinds of data constantly.
Five discussion leaders examined the area, discussing seismic, logging and measurements, lab analysis of temperature effect on relative permeability and rock stresses in understanding the carbonate.
Dynamic Well Testing
This session examined
- Four well-testing interpretation models based on triple porosity and a discrete well-testing model based on a discrete fracture network (DFN).
- Eight flow models that simultaneously occurred in the Jujo-Tecominoacan field.
- The causes of absence of double porosity behavior in fractured carbonate reservoirs: high wellbore storage, small matrix block, and multiblock sizes.
Two discussion leaders reviewed dynamic well testing. One focused on four well-testing interpretation models that have been established based on triple porosity continuous media that include well connects with cavities, well connects with cavities and fractures, well connects with pores, cavities, and fractures, and nonlinear Darcy’s flow in the near wellbore. Also discussed was a discrete well-testing model that has been established based on a DFN model. Cinco-Ley discussed highly homogeneous and strong anisotropy in dynamic performance in fractured carbonate reservoirs, and eight flow models (such as a compartmentalized reservoir model, dual porosity model, etc.) that simultaneously occurred in the Jujo-Tecominoacan field. He interpreted the reasons for the absence of double porosity behavior in fractured carbonate reservoirs as high wellbore storage, small matrix block, or multiblock sizes.
Carbonate Reservoir Simulation Modeling
This session studied the mathematic model and simulation method for carbonate fracture vug. Two problems were raised:
- How to get an effective and efficient model as soon as possible.
- How to minimize the uncertainty of the geology model.
One suggestion was to make several geology models at the same time and then choose the one most consistent with production. A good reservoir characterization-to-model fluid flow through matrix, fractures, and vugs will reduce uncertainty in reservoir simulation, especially to simulate enhanced oil recovery with chemical injection since that is expensive. Naturally fractured reservoirs with low matrix permeability can be simplified as single porosity models in reservoir simulation.
Sweet Spots, Well Drilling, and Trajectory
Pressurized mud cap drilling technology could be used in lost carbonate formations to improve operational efficiency and safety. An electromagnetic lost circulation detector has high accuracy in thief zone identification. Cross-link bridging plugging technology can provide sealing pressure for fractures.
Presentations focused on pressurized mud cap drilling for carbonate wells, thief zone identification, and formation pressure prediction in the next session. On carbonate reservoir drilling in Malaysia, the practices of pressurized mud cap drilling technology, well engineering architecture, and formation evaluation strategies were discussed.
Other topics discussed included a new method of thief zone identification using electromagnetic detectors, hydraulic analysis of fracture width, and cross-link bridging plugging technology for fractures. Another topic was pore pressure prediction using logging data, seismic attributes individually, or using pore volume and pore fluid volume together during drilling, as well as their applications in Iran and Sichuan, China. Compared with well-testing data, those methods could enhance prediction accuracy significantly.
Completions and Stimulation
The next session reviewed how
- Proppant fracturing technology is proving better than acidizing in PetroChina’s Tarim high-pressure/high-temperature carbonate reservoir.
- Ground cross-linked acid fracturing has been successfully applied at Sinopec’s Tahe oil field to get longer etched fracture length.
- Novel stimulation techniques have been developed for carbonate reservoirs, such as self-diverting fluid based on viscoelastic surfactant and degradable fiber for moderate to high permeability carbonate reservoirs.