As Effective and More Economic Than Diverters

As Effective and More Economic Than Diverters

CHALLENGE
In effort to increase well performance via stimulation efficiency, an active Permian operator (“Operator”) selected Quantico Energy Solutions (“QES”) to provide horizontal well log data for an completion project in the Wolfcamp formation in Reeves County, Texas in the Permian basin.  The Operator’s objective was to optimize the effectiveness of the fracture treatment, thus increase hydrocarbon recovery, by utilizing plastic particulate diverter systems in each frac stage where 125 pound diverter slug was added to each frac stage. Diverter technology is designed to “plug” open perforations that are accepting frac fluid and divert bottom hole fracturing energy (pressure and rate) to perforations that are not being treated, thus increase the stimulated reservoir volume for the stage. Thus, more perforations should be stimulated and lead to increased production.  The challenge was to determine the costs versus benefits of the diverter in a wellbore where stress between perforation clusters had been minimized by optimizing perforation placement.

Conventional open-hole logging solutions would have incurred a minimum cost of $200,000 in addition to $75,000 in costs for the diverter material and increased pumping operational costs. Mud logging and cuttings data provide a geological/lithological description of the wellbore, but do not address the rock mechanics issues. The performance of the QES engineered perforation cluster depth locations were evaluated using pressure pumping and post frac modeling by StrataGen who was contracted by the Operator.  

SOLUTION
QES provided an array of geomechanical logs (“QFrac”) that were derived from the well’s drilling data and enabled a 80% savings to the Operator when compared to the cost of conventional e-line small diameter tool string pump down logging operations. The QFrac logs were generated within a few days of receiving the existing drilling data from the Operator, which allowed sufficient time for the pressure pumping service company and the stimulation consulting company to optimize the treatment designs based on geo-mechanical properties along the 4,500 ft. length of the lateral section.  The necessary datasets were integrated as standard well log presentations and associated digital data files.  QES’ solution did not require any tools to be placed into the wellbore thus reducing lost-in-hole and operational risk for the Operator.

QFrac provided an array of geomechanical properties, presented in a well log format upon which decisions to optimize perforation cluster placement were based.  The stage locations remained geometric consistent with prior practices. These optimized perforation cluster placements took into consideration the key parameters that affect the quality of the hydraulic stimulation treatment such as horizontal stress (Sh), porosity (EPOR/DPOR), brittleness, and permeability index.  Having QFrac during the planning phase enabled the Operator to effectively evaluate various treatment parameters such as (a) placing perforation clusters into rock with similar stress; (b) optimization of the injection rate based on Sh and Young’s moduli; and (c) selecting the appropriate fluid systems based on anticipated leak-off and proppant concentration.

The 29 planned stages were executed as designed with no major operational issues of note. The pressure pumping data was analyzed for each stage (see Figure 1) and concluded that relative to the predicted stress, the recorded stress exhibited average variance of only 5%. 90% of the stages exhibited stress variance less than 10%.

Figure 1.  Horizontal Stress (Measured vs QES Prediction)
Diverters - Fig1

Figure 2.  Horizontal Stress across Stage
Diverters - Fig2

An independent analysis conducted by StrataGen determined the number of perforations that opened before and after the diverter was applied. On average across 29 stages, the diverter sealed off only 2 perforations per stage (see Figure 3). The analysis concluded that the effectiveness of the diverter was marginal, when the perforation cluster locations were selected using minimal stress variations (Sh) for each stage. Had a geometric completion strategy for selecting perforation cluster locations for each stage been utilized, the effectiveness of the diverter may have been efficient.

Figure 3.  Perforations Sealed Due to Diversion
Diverters - Fig3

By placing the perforations in areas of similar stress (see Figure 4), the stress contrasts were significantly reduced for each stage compared to the geometric design typically implemented when little or no horizontal data is available. The optimized perforation placement resulted in more perforations opening.  As a result, the relatively small slug of particulate materials were flushed out of the larger number of perforations that had already been opened by the initial frac operations, before they could plug what normally, in a geometric scenario, would have been a smaller number of open perforations. Overall, the diverter had minimal positive observed effects on plugging perforations. 

Figure 4.  Maximum Stress Contrast across Perforation Clusters
Diverters - Fig4

VALUE
QES provided the Operator with critical formation data for this horizontal well that would have been cost prohibitive to log using conventional methods. Having minimal stress variation between perforation clusters improved the hydraulic fracturing treatment. The pressure pumping data (Fig. 5) shows the effectiveness of the perforation clusters accepting fracture fluids. On the horizontal time axis at approximately 23 minutes, the rapid increase in both the surface pressure (white curve upper graph) and the injection rate (yellow curve) rapidly increase, and then at approximately 25 minutes the rapid decrease in the surface pressure and stabilized injection rate indicates effective perforation opening. The later in time data from 34 to 82 minutes illustrates effective in zone fracture propagation identified with near constant pressure and rate, with increasing proppant concentration (lower graph red curve). The decrease in pressure and rate between 84 and 88 minutes is the placement of the slug of diverter material. The rapid increases to a stabilized injection pressure and rate, with increasing proppant concentration after the placement of the diverter slug indicate minimal effectiveness of the diverter. Analysis of the pressure pumping data for all 29 frac stages indicated no additional perforations were opened after the diverter was injected except in stage 4 which showed to have added an additional 2 opened perforations.

Figure 5.  Representative Stage Treatment Data
Diverters - Fig5

Optimization of perforation placement in zones of minimal stress contrast by using QFrac data requires no personnel or equipment on location and also no tools in the wellbore.  Using diverter requires (i) additional time due to reducing injection rate to as low as ten (10) bpm, (ii) an additional amount of chemicals and (iii) a dedicated blending unit and frac pumps that are isolated from the primary frac equipment.  Each of the listed items increases operational risk exposure and costs.  Assuming average clean fluid (no sand) pumping cost per minute (80 bpm and slickwater fluid) being $388/min and the average time spent dropping diverter being 6 minutes would equate to $2,325 spent for additional pumping diverter. Over 29 stages, an estimate of operational cost would be $67,512.

Whereas diverters are used to apply treating pressures to areas of variable stress, QES allowed the Operator to reduce variable stresses in each stage prior to the frac. The stage could be treated conventionally with no design injection rate changes or equipment switching necessary.

Total cost reduction delivered to Operator:

A) $75,000 saved on the cost of using diverter
B) $67,000 saved from additional pumping time and additional chemicals
Total = $142,000 total savings on a single well