Introduction

The Chandler Perforation Plot (CPP) has been developed to improve identification and characterization of potentially productive zones within a hydrocarbon bearing reservoir penetrated by a directionally drilled well bore. The method is designed to capture and measure hydrocarbons entrapped in the formation, and can be used to optimize drilling and completion effectiveness related to geo-steering, perforating, stimulating and producing a new well bore.

The Problem

Mud gas detection systems are commonly utilized in vertical well bores to detect and identify hydrocarbons from circulating mud systems. However, mud gas detection systems and their resulting gas curves do not perform equivalently in vertical and horizontal well bores. The total gas curve in a vertical well bore responds to an increase in gas that is typically circulated up from a drilling break. Additional information available may include background gas and rate of penetration before, during, and after the drilling break. The information gathered can then be used to determine the composition of the gas and its placement within the formation.

There are several reasons why a conventional gas curve taken from a circulating mud system in a lateral well bore will not have the same performance or appearance as a gas curve taken from a vertical well bore. Gas increases in lateral wells often lack the before, during, and after drill times since the formation is relatively homogeneous, resulting in flat unresponsive rate of penetration. The well site geologist is left with little information to interpret the gas increase and determine an accurate placement along the well bore.

Vertical well bores penetrate hydrocarbon bearing zones through a short interval, so gas increases and then decreases after a potential pay zone has been drilled. Lateral wells remain in the zone over long intervals which may cause elevated background gas due to produced gas, elevated connection gas, trip gas, and survey gas.

Another problem in lateral wells is the fluctuation of drill time during the sliding process. Slower penetration rates while sliding result in less gas being released from the matrix. High penetration rates result in artificially high gas units. Both scenarios mask fluctuations that might otherwise provide valuable and useful information.

The Solution

A new and promising method for evaluating lateral wells has been developed where gas can be extracted directly from drill cuttings and the data points plotted on a gas curve. The end result is a gas curve that is not affected by high contents of background gas. This process has been named the Chandler Perforation Plot (CPP) and is currently in the Patent Pending process with the United States government. The CPP is a collection of data directly from specific intervals in the formation after removing any contamination and other irrelevant contributing factors to the total gas curve. This unique process has been performed successfully in field trials of limestone, shale and sandstone reservoirs although the exact method remains proprietary.

Procedure

To avoid contamination, quality control must begin with the collection of samples at the well site and continue throughout the procedure. Once at the lab, all samples to be evaluated go through particle size distribution which is dependent on rock type and bit selection. This is performed so that the matrix size is generally the same because sorting size is critical to produce consistent and comparable results from the analysis.

Since the rock samples contain entrapped hydrocarbon material, it is also important to accurately measure and test each sample. The analysis of samples from different locations along the well bore must be performed in identical conditions. This step is crucial to facilitate the comparison of samples in order to accurately locate the potentially productive zones within the formation.

General Procedure:

1. Drill well bore into hydrocarbon bearing formation at an angle >49°

2. Collect drill cuttings from well bore

3. Release entrapped hydrocarbons from rock samples

4. Capture released hydrocarbons

5. Measure one or more properties of released hydrocarbons

6. Organize results and plot on Chandler Perforation Plot (CPP)

7. Distribute results to end user

Advantages of CPP

The CPP offers many advantages compared to gas measurements taken from a circulating mud system because the gas recorded on a CPP is recovered directly from cuttings from known depths. This results in more accurate gas measurements with use of the CPP methodology for several reasons.

When live oil is added to decrease drag in the hole, it elevates total gas measurements taken from the mud system; however, the CPP is not affected by the addition of live oil. The CPP provides a high performance gas curve, free of produced gas, background gas, connection gas, trip gas and is not affected by hydrostatic pressure. The conventional gas curve is affected by the rate of penetration whereas the CPP is only affected by the amount of gas in the matrix. The CPP is an ideal tool to determine what section of a lateral well bore contains the highest concentrations of trapped hydrocarbons and what part contains noncommercial to marginal concentrations. This can help cut operating costs significantly with regards to stimulation because the only zones requiring stimulation are those that show good response on the CPP. Additionally, the CPP can be utilized to stay ‘in-zone’ in order to increase potential production and effectively drain the target reservoir. In field trials this methodology was also indicative of depletion when lateral well bores were spaced too closely.

A True Unit of Gas

A conventional gas curve is displayed in a generic measurement known as a unit, which corresponds to relative increases and decreases. Numerous variables affect the measurement of gas units including placement of the agitator level in the possum belly, different voltages used to burn the gas and the volume of carrier air crossing the filament. Other factors are also at work, but the bottom line is there is no efficient standardization for the many hot wires in use today.

The CPP measurements are not affected by these previously mentioned factors. When a data point of 50 is collected from a sample that is sieved, weighed to the standard, and burned through a calibrated hot wire; additional testing of the sample will result in the same value. Thus 50 units now means something when compared to another sample of 10 units, namely there is five times more gas in the first sample than in the second. With the CPP and its methodology there is no doubt where the gas increases are coming from and how they compare to one another.

Once commercial and noncommercial wells have been established in a given area, the corresponding CPP measurements for productive vs. nonproductive zones in a well bore and from well to well can be better calibrated. As parameters are established and the database is developed, this methodology will lead to improved drilling and stimulation decisions, and can be used with real time data from the well site to identify when you are in zone allowing geo-steering decisions to be made on the fly.

Conclusion

The CPP is superior to gas measurements taken from a circulating mud system. The CPP gas curve is free of all contamination from the well bore and can be used without interpretation or confusion. The gas is in the rocks and the CPP is a more accurate measurement than other gas curves. Chromatography data can be achieved in the same way to define the liquid content of the section if so desired. The data base assembled from this methodology will increase in value as more and more wells utilize the CPP in a given region. Eventually this data base may contribute to decisions to set production casing, especially in the case of depletion. The sermon is in the stones, and it always will be.