Consideration of proppant economics before selecting a proppant is of substantial importance to stimulation design in the Eagle Ford formation. To operators, the value of proppant is the most important factor of proppant economics. Using the cost ratio between proppants and conductivity adjustments are just two ways to determine the value of proppant. When one of these methodologies is used to determine which proppant has the greatest value, operators can achieve the biggest “bang for their buck.” Conductivity and how sands are impacted at closure stress greater than 6,000 psi also must be examined.

Some operators in the Eagle Ford formation have pumped sands at 2,743 m to 3,353 m (9,000 ft to 11,000 ft) true vertical depth (TVD), observing the effects of shattered sand as it is later cleaned out of surface equipment every month. Currently, some operators are pumping sand at 3,810 m (12,500 ft) TVD in the Eagle Ford formation with a fracture gradient of ~1.05 psi/ft. At 12,000 psi of closure stress, more than 95% of all the white sand pumped has been damaged. To put this in perspective, on a typical well in the Eagle Ford formation, proppant can cost up to US $1 million. If 95% of the proppant has been damaged, then $950,000 has been spent on damaged white sand. Within any project, the limitations should be known upfront. In the case of the Eagle Ford formation a wide range of depths can be examined, as the Eagle Ford formation has depths from 610 m to 6,096 m (2,000 ft to 20,000 ft) TVD, depending on the geographical location. Most of the drilling in the Eagle Ford formation occurs at depths ranging from 1,219 m to 4,877 m (4,000 ft to 16,000 ft) TVD.

Proppant considerations

The two most commonly examined properties of proppant are fracture conductivity and price. Proppant damage is one of the mechanisms that will reduce conductivity. Determining the correct stress initially is critical to proppant selection and maintaining conductivity once the proppant has been placed. Because of criticism with respect to the accuracy of the equation for closure stress on proppant in the fracture, in this work it is assumed for our comparisons that the minimum in situ horizontal stress is the closure stress on the proppant.

graph- fracture conductivities

The output proppant pack conductivity of an Eagle Ford completion at 4,206 m (13,800 ft) TVD was simulated to compare the fracture conductivities of white sand and VersaProp using different amounts of proppant. (Image courtesy of Halliburton)

Baseline conductivities of sample proppants are used because using dynamic conductivities can lead to endless results from an infinite number of possibilities. In the interest of keeping it simple, all of the conductivity values are taken from the same third-party commercial testing laboratory. It can be assumed that, by using a single testing vendor’s data, errors in the conductivities are minimized.

Method 1: Cost ratio between proppants

The first method for proppant value can be determined by dividing the proppant’s conductivity by the prop-pant’s cost. This calculation will result in the actual value of the proppant.

Method 2: Conductivity adjustments

The second method is considerably more involved. To determine a dollar-for-dollar comparison between proppants, a process of generating adjusted conductivities is necessary. Based on the equations used, proppant conductivity has a linear proportionality to the concentration of the proppant pack. In general, for round proppants, this relationship is generally true for most laboratory measurements when no additional damaging components are present. In other words, the conductivity of a proppant pack at a concentration of 4.88 kg/sq m (1 lbm/sq ft) is about half of the conductivity of that with a proppant pack at a concentration of 9.77 kg/sq m (2 lbm/sq ft), as the resultant pack widths are proportional to mass per unit area. Knowing that conductivity and concentration have a linear relationship at similar stresses is the key to calculating the second method.

The next part of the second method is determining the cost ratio between the subject proppant (white sand) and the other proppants of interest at a specific closure stress value. The cost ratio is defined as the cost of proppant of interest over the cost of subject prop-pant. Because conductivity and concentration have a linear relationship, one can multiply the conductivity by the inverse of the cost ratio to determine the new effective proppant concentration that is a dollar-for-dollar equivalent.

In other words, if proppant B is two times as expensive, what would the conductivity be if only half as much was used?

The idea is to predict what the conductivity would be if a dollar-for-dollar equivalent existed between prop-pants. This is a completely new way of thinking about proppants and as these have traditionally been viewed on a pound-for-pound basis.

An example of the second method would be to choose two proppants and record the conductivities at the same stress (7,000 psi at 9.77 kg/sq m) and the cost per kilogram. The cost ratio can be determined by dividing the new proppant’s cost over the original proppant’s cost – for instance,the cost of a ceramic divided by the cost of white sand, which typically is a ratio of 2:3.

The conductivities recorded were at a proppant pack concentration of 9.77 kg/sq m. Dividing this concentration by the cost ratio would result in the concentration of the new proppant to effectively normalize the cost between the two proppants to focus on the conductivities. With the new concentrations for the two prop-pants, a variety of software applications can be used to determine the proppant pack conductivity for the new proppant.

It is possible that better proppants could yield higher conductivities using a fraction of the amount of white sand. Even at stresses between 4,000 psi and 7,000 psi, white sand appears to have a higher value.

Design simulation

This economic analysis was designed to find the optimal proppant for its cost. If millions of dollars are spent on proppant, why not achieve the highest conductivity possible at a comparable cost? This question implies that possibly few operators have spent the time or resources to determine the best option with respect to proppants.

The simple reasoning of many is that 1.81 kg (4 lbm) is better than 0.91 kg (2 lbm) because more proppant should equal better production. Taking the proppant damage into consideration reveals that the sands will have more than 90% damage at 8,000 psi, while most of the other coated and ceramic proppants are still below 75% damage. This is important to consider as damage affects actual conductivity; it also has an impact on how much proppant might later be produced to surface. In the event that this analysis has resulted in a new proppant being chosen, small changes to the pump schedule will help ease the transition.

To maintain the current fracture geometry, it is assumed that the amount of fluid injected into a reservoir will remain the same; in other words, all stage volumes previously used will be the same. This will help reduce the number of variables when comparing the new design to offsets previously completed. Second, it is important to reduce the proppant concentrations on the job until the job proppant total mass is cost-comparable to the original job design. The intent is to spend the same amount of money and achieve higher conductivity. This new design can be used in conjunction with hydraulic fracture modeling software to verify the results and support the analysis. The output proppant pack conductivity of an Eagle Ford completion at 4,206 m (13,800 ft) TVD is shown in the figure. The conductivity coloring shown on all the images is using the same scale (0 to 600 mD-ft). The conductivity in the simulator output appears to be lower than other conductivity tests, so it is assumed that the simulator is factoring similar damages as the dynamic damage effect is turned off in the simulator.

The fluid system used in the simulation is 18.14-kg (40-lbm) crosslinked guar at 177°C (350°F) bottomhole static temperature. This fluid system is not recommended at this temperature mainly because of premature proppant settling, which can be observed from the results.

The sand simulation clearly shows it to be on the low end of the conductivity scale. Nowhere in the fracture does the simulation predict more than 120 mD-ft. Even if the pumped concentration of white sand is doubled from 0.48 kg/l to 0.96 kg/l (4 lbm/gal to 8 lbm/gal), conductivities higher than 180 mD-ft will not be achieved. To determine how this compares in terms of value, VersaProp proppant seems to be a better choice for this job than white sand. While there are other proppants to use in this example, the pricing of ceramics and bauxites has a great impact on this analysis, and any changes could qualify a different proppant as the best choice.

When using VersaProp proppant at 0.24 kg/L (2 lbm/ gal), the simulator figure shows a little more than 300 mD-ft. Even using this proppant at a quarter of the original amount still allows stimulations at higher conductivities than the original amount of white sand at this stress level. By matching up the pump schedules for 0.48 kg/L (4 lbm/gal) and using this proppant, the simulator shows very high conductivities.