Exploration

Drilling blind in complex geology?

Using near real time mud logging to unscramble correlation

Val Neshyba, Houston Exploration Co.; Carl Greer, Precision Well Logging, Houston

Consider the following assignment. You are to develop a field with several potentially productive sands, where chances are great that each well will be somewhat, or totally, different from adjacent wells. Then drill where there is inconclusive evidence as to the exact placement, presence and/or producibility of targeted sands. Finally, drill through potentially productive sections without going deeper than the last zone of interest, and without allowing a surprise encounter with blow-out range gas or other expensive, rig-time-consuming problems.

The above task is difficult and frustrating if drilling blind through most of the well. Due to the difficulty of correlation, these wells, although within a field, drill like rank wildcats. In such prospects, real-time mud logging to develop correlation is a priority among the evaluation services available.

DECISION TIME

Geologists faced with the responsibility of successfully drilling and completing wells in a field such as this, and doing it with optimum expense, often turn to mud logging. Some consider it essential. Although it is tempting on the part of operating company management to cut services to reduce drilling costs, mud logging provides dependable, near-real-time information to consistently drill fast, accurate and safe. In the overall operational scheme, mud logging is very cost-effective in reducing downtime and excessive drilling, especially in aggressive development situations.

MWD and LWD logs solve many correlation problems with adjacent wells during drilling. However, the economics are often negative when using these logs, except in the most demanding, expensive drilling prospects, making them unrealistically costly for medium-depth wells. Add to this the complication of productive sands presenting low electric-log resistivity and the adverse effects of oil base mud, and the need for mud logging becomes obvious. A good example is the Lobo trend of the Lower Wilcox in South Texas.

CASE HISTORY

The pressured Lobo and the Midway make up the Lower Wilcox section in Webb and Zapata counties of South Texas. The Lobo is Paleocene in age and comprises multiple sand and shale packages representing a deltaic sequence that was grossly deformed after deposition due to massive slumping. It is 1,500 ft of extremely faulted section. Each sand package has defined fairways within the trend. The Lobo 6 sand package represents the largest and most widespread depositional pulse and is commonly used to define the extent of the basin. To the trained eye, the Lobo 6 is easily recognizable on a mud log and is key to projecting total well depth, reached just short of abnormal pressure, Fig. 1.

Fig. 1. Simplified Lobo schematic.

Fig. 2. Mud log of a well drilled in the Lower Wilcox/Lobo.

Fig. 2. Mud log of a well drilled in the Lower Wilcox/Lobo.

The Midway Shale underlies the Lobo. Its highly elastic nature was the catalyst that allowed the slump to occur. Listric faulting in the Lobo bottomed out along the base of the Midway where it met the Cretaceous Unconformity. The lowest portions of the Midway (re-deposited Cretaceous sediments) and the Upper Cretaceous are pressured, fine grain, gas bearing, tight, calcareous sediment.

Interpreting the Lobo. Interpreting the Lobo section is intensive work. 3D seismic has been very beneficial in bringing out structural complexities of the trend. Abundant remaining upside lies within the small, untapped blocks in areas of the trend that challenge the resolution of current technologies. Still, especially with the untrained eye, interpretation errors occur and the geoscientist, encountering unexpected faulting, can overshoot the Lobo and subsequently drill into the pressured underlying sediments. Without a mud log providing near real-time availability of correlatable data, he may think he has found a fully charged Lobo 6 block.

In this scenario, the drilling rig would experience a gas kick and be forced to increase mud weight to continue drilling. Four or five hundred feet later, it becomes obvious that the Lower Midway or Cretaceous has been penetrated, the well is at risk, and rig time is mounting. Simply put, the mud logger would have detected the problem early due to the absence of sand lithology appearing with the gas.

Successful exploration/logging in the Lobo. Multiple sands and faults in a mature, fracture-stimulated producing trend like the Lobo invite the additional risk of partial depletion. The Lobo also has its share of low-resistivity pay zones. These complications can challenge a company to make fast and accurate decisions concerning when to call for pipe as TD approaches. One successful exploration company based their decision strictly on oil/gas shows and target definitions gained from the mud log and interpreted by an experienced geologist. This procedure cut drilling costs by getting production casing to the rig early, reducing overall rig time, Fig. 2.

Drilling the Lobo for production requires two strings of casing. Increasing formation pressure is first detected in the Middle Wilcox 1,000 ft or more above the Lower Wilcox (Lobo Unconformity). Before reaching this pressure, there are excellent electric log correlation markers, found in offset wells, such as the Wilcox “A” sand (also called the Big Blue) in the middle Wilcox. Big Blue is the first sand in the middle Wilcox. The depth it appears, with respect to known formation thickness of other wells, is used to project the upcoming intermediate casing point (ICP). The Big Blue and other less distinct markers are revealed in the mud log. Logs are emailed to the geologist as the sand markers are drilled. The mud log is also used to fine-tune the final ICP by targeting a clean shale section for casing shoe placement. Proper placement of the ICP is crucial to successfully drilling the remainder of the well.

CONCLUSION

Mud logging is a inexact term for the service described here, since just part of the analysis is derived from the drilling fluid. Even the phrase “real-time logging” is inexact, as it would be true only for the rate of penetration, since the other parts of the log are recorded after a lag time, that is, after the drilling of a section has occurred and the information-laden drilling fluid is pumped up for analysis. This lag time could vary from a few minutes to one or two hours, depending on depth and the annular velocity of the drilling fluid.

The geological complexities of the Lobo challenge the resolution of sophisticated data, 3D seismic tools, and MWD/LWD abilities. It is very much an interpretive environment. An experienced geologist in the Lobo should consider mud logging to be the most cost-effective in reducing down time and excessive drilling, especially during periods of aggressive development. For example, using the mud log for selection of the intermediate pipe point provides a clean shale for the casing shoe, located at the most advantageous point in the section. The mud log generated by an experienced logger, as the well is drilled, provides a valuable correlation tool that accurately resembles offset electric well logs. This enables the geoscientist to predict targeted points to within a few feet and unscramble complex geology before TD is reached. No formation evaluation method is 100% accurate and consistent, including mud logging. However, the Lobo of South Texas is an example of the best use of near-real-time mud logging.  

THE AUTHORS
	Val Neshyba earned a BS degree in geology from the University of Houston in 1975, and a petroleum engineering degree from Texas A&M University in 1979. He has 23 years of Gulf Coast experience, focusing 13 years on the South Texas Wilcox/Lobo trends. He is an exploitationist for the Houston Exploration Company.
	Carl Greer completed various coursework at Texas Arlington State College in 1946. He has 50 years’ experience operating mud logging services in the southern and southwestern US and offshore. He is president of Precision Well Logging, Inc., Houston.