Depleted Sand Drilling

Introduction

As the search for oil and gas continues it is apparent that the majority of the "easy-to-get-to" shallow productive pay sands have been drilled. Some have been producing for many years. With the introduction of new technology such as 3-D seismic, new potential reservoirs have been identified. Many of these potential reservoirs are pay sands located beneath the older producing field wells. In order to reach them, drilling through these older produced sands is necessary. Many of these produced sands' reservoir pressures have been reduced. Some are as low as 1.0 ppg equivalent. Conversely, the mud weight required to balance reservoir pressures below the depleted sand may be considerably higher. Consequently, the resulting differential pressures across the depleted sands can be considerably higher than standard drilling techniques can address. Problems encountered when drilling through these depleted sands are primarily stuck pipe, through differential sticking, and loss of returns. Properly addressing these problems can mean the difference between successfully reaching the objective sand and loss of the wellbore. Drilling fluids play a major role in the success or failure of these type of wells. Here we will discuss the use of a permeability plugging apparatus to help design fluid properties and successfully drill these critical well types.

Permeability Plugging Apparatus

Filtration properties of drilling fluids as currently measured (API and HTHP fluid loss) do not accurately reflect downhole filtration. These methods are merely measurements of trends under static conditions. Actual filtration occurs downhole under both static and dynamic conditions. Additionally, these test methods do not reflect the pressures encountered at the zone of interest. Filtration is measured in these tests using Whatman 50 filter paper which is not representative of downhole formation faces. Although there is equipment currently in use which can more closely model downhole static and dynamic conditions, they are large, expensive, and incapable of being transported to the field for wellsite use.

The permeability plugging apparatus or tester (PPT) was designed and built as a joint effort between a drilling fluids company and a major operator. The goal was to provide a portable testing device capable of more closely modeling downhole conditions of temperature and pressure differential. Utilizing the basic design of the standard HTHP cell and heating jacket, modifications were made to both allowing for application of higher pressures to the test cell. Estimated bottom hole temperatures are applied to the fluid utilizing the heating jacket.

The old standard Whatman 50 filter paper was replaced with an alloxite disc of known porosity and permeability. Pressure (2500-4000 psi) is applied to the cell utilizing a hydraulic pump which pumps oil to the cell through a float. A piston exerts a measured amount of pressure against the drilling fluid within the cell. The cell is run inverted (alloxite disk on top) to negate the effects of solids settling during heating. Fluid pressure is exerted on the alloxite disk (modeling the depleted sand permeability and porosity) on the opposite end of the cell (bottom). Mud and/or filtrate is collected and measured from a reservoir behind the disk at the top of the cell. Permeability plugging test (PPT) values are then used to determine the sealing capability of the drilling fluid. The lower the spurt and total filtrate values, the better the sealing ability. As permeability is decreased, so are the effects of the differential.

The PPT Test

The first step in running the PPT is to select the disc which most closely models the formation. Common disk permeability's range from 100 md to 100 darcies. It is common practice to use discs with permeabilities 2-3 times greater than the formation of interest. The result is greater assurance that the drilling fluid has the ability to effect a seal across the formation.

The disc is soaked for 5 minutes in water (for water based muds), base synthetic (for SBM's), or diesel (for oil based muds). Soaking fills up the pore spaces in the disc to prevent erroneously low spurt losses.

The disk is loaded into the cell and the cell then loaded into the heating jacket. 500 psi of pressure is applied to the cell while it is heated to the desired temperature. After reaching temperature, additional pressure is applied to reach the desired differential (2,000-4000 psi). The initial spurt loss is then taken and recorded. Spurt loss is defined as the amount of mud and/or filtrate recovered from the filtrate collector immediately after the differential pressure is applied until the immediate flow of fluid through permeable disk stops and gas from the back pressure regulator starts to blow out the collector. After 30 minutes, making sure that desired pressure is maintained on the cell, the filtrate volume is collected and recorded. The filtrate volume in milliliters (not including the spurt loss) is doubled and reported as the filtrate volume. For example, if the spurt volume was 3.5 ml and the filtrate volume collected in 30 minutes was 6.8 ml, the results would be reported as:

Spurt = 3.5
Filtrate = 13.6
Total Filtrate = 17.1.

Objectives

Successfully drilling through depleted sands requires an understanding of differential sticking. It is important to understand that permeability or the ability of fluids to flow through a medium, in this case the formation sand, is key. When drilling fluids under pressure contact the formation and permeability exists, mud cake deposition begins. Poorly treated or contaminated muds produce high fluid losses and thereby thick filter cakes. These thick filter cakes. lead to greater surface area contact between the drill pipe and the filter cake. The differential pressure across the formation face forces the pipe against the borehole rendering the drill string stuck. The force required to pull free is exponential to the area of contact between the pipe and the wall cake. Therefore, minimization of cake thickness and thereby surface area contact reduces the likelihood of stuck pipe.

Through utilization of the PPT, drilling fluids can be designed to:

1. minimize spurt loss
2. minimize total fluid loss
3. minimize cake thickness

It should be remembered that two fluids can yield similar fluid losses in the PPT, yet cake thickness can be quite different depending upon the materials used to achieve them. Cake sizes should always be viewed, recorded, and considered when evaluating the quality of a drilling fluid for drilling a depleted zone.

The particle size distribution of the solids contained in a drilling fluid has a direct affect on the sealing ability achieved. When the particle sizes are close to and smaller than the pore diameter in the depleted sand, sealing of the pore spaces occurs, permeability is reduced, wallcake buildup and corresponding pipe to wallcake contact area is reduced, and the risk of differentially stuck pipe is greatly diminished.

Guidelines for Depleted Sand Drilling

• Determine lowest pore pressure of sand to be drilled.
• Determine highest mud weight required in that interval.
• Calculate maximum pressure differential to be encountered.
• Obtain, when available, any porosity and permeability data of the depleted sand.
• Select an appropriate alloxite disc for PPT testing.
• For planning purposes, prepare a base mud sample with anticipated product concentrations for drilling the depleted zone. Run PPT tests on this sample to roughly estimate material requirements for drilling the depleted interval.
• When drilling commences , run PPT tests on the system fluid to get a base line spurt and total filtrate.
• Send a sample in to the lab to run a particle size distribution analysis to determine the particle size range to target for plugging efficiency.
• Utilizing the above data, pilot test with specific concentrations of products (i.e. CaCO3, lignite, sulfonated asphalt, gilsonite, fibers, lubricant, etc) to target spurt loss reductions and positively impact total filtrate.
• Minimizing cake thickness should be a key element in evaluating treatment effectiveness. Do not evaluate tests based upon spurt loss alone.
• The PPT well site pilot testing should be performed to allow ample time to get the required materials to the drilling location.
• Prior to penetrating the depleted sand, prepare a large volume pill (100-200 bbls) using system mud and high concentrations of loss circulation material. Be prepared to pump this pill at the first sign of loss circulation.
• Monitor static shears and maintain shear values below 250 lbs/100 ft2 . This will minimize the surge pressures during tripping operations and minimize the risk of additional imposed pressures on the depleted zone.
• After drilling and just prior to running casing, spot a spill with additional particulate across the zone of depletion to minimize the risk of sticking casing.

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