**7. Discussion of five deficiencies**

Hydraulic fractures must achieve two primary objectives. They must:

Hydraulic fractures must achieve two primary objectives. They must:

fractured interval, along with three plausible production matches.

**2.** Provide a durable conduit for hydrocarbons to flow to the well with acceptable pressure

Complex, branching fractures do an excellent job of touching rock. However, they challenge our ability to place a commensurate degree of conductivity. Branching, complex features are

The fifth thing we don't want to know about fractures is that it is nearly impossible to identify the deficiencies when analyzing production data from a single well. Figure 9 shows the production history (decline curve and cumulative production) from a single fractured interval,

blamed on the geology – with no irrefutable proof that the fracture was insufficient.

common expectations, even if the fractures are planar and fully propped.

Figure 9. With a single well, the production history can be matched with a nearly infinite combination of plausible fracture and reservoir

Actual production data Long Frac, Low Conductivity Medium Frac, Low Conductivity

Short Frac, High Conductivity, Reservoir Boundaries

• History matching of production is surprisingly non-unique.

• Too many "knobs" available to tweak • We can always blame it on the geology

500' Xf, 20 md-ft, 0.5 uD perm, 23 Acres 4:1 aspect ratio 100' Xf, 20 md-ft, 5 uD perm, 11 Acres 4:1 aspect ratio 50' Xf, 6000 md-ft, 10 uD perm, 7 Acres 4:1 aspect ratio

**Figure 9.** With a single well, the production history can be matched with a nearly infinite combination of plausible

From a single decline curve, we cannot uniquely determine whether the fracture is short and "infinitely conductive," or long with more significant pressure losses. We cannot prove from a decline curve whether the fracture was simple or complex in geometry. We cannot prove whether the fracture conductivity was constant or degrading. Most engineers attempt to match the data with an analytic solution or a numerical simulator that presumes the frac is fully packed with proppant throughout, providing uniform and durable flow capacity without collapse of poorly propped sections. Note that with this approach an engineer can continue to reinforce any existing misconceptions. Fracs can be interpreted to be long or short. Disap‐

Cumulative Production (MMscf)

more significant pressure losses. We cannot prove from a decline curve whether the fracture was simple or complex in geometry.

There are certainly more than five deficiencies in our stimulation designs and our techniques to analyze well production. However,

1. Hydrocarbons move in a complex manner within propped fractures, increasing the pressure losses by 50 to 1000-fold over

2. Fracture conductivity is not constant. Lab data suggest that all conventional proppant types suffer continued crush and

3. Reservoirs are laminated and compartmentalized. Especially with horizontal drilling, ultimate recovery is far more dependent on fracture continuity through laminations than in vertical wells in which each prospective layer can be perforated and individually stimulated. With low perm reservoirs, significantly longer well life (and proppant durability)

4. Fractures develop varying degrees of complexity. This is both good and bad. Reservoir contact is increased as fractures branch, twist, and energize pre-existing planes of weakness. However, this complexity challenges our ability to place a

often ineffectively propped, with risk of insufficient conductivity and continuity.

**1.** Touch rock (contact hydrocarbons)

continuity [adapted from 17]

88 Effective and Sustainable Hydraulic Fracturing

losses (sufficient conductivity)

1. Touch rock (contact hydrocarbons)

**6. Non-unique interpretations**

continuity.

descriptions [18, 19]

fracture and reservoir descriptions [18, 19]

Stage Production (mcfd)

along with three plausible production matches.

**7. Discussion of five deficiencies** 

0 100 200 300 400 500 600 Production Days

the five issues described in this paper include:

compaction over time.

will be required to drain the available reserves.

**6. Non-unique interpretations** 

2. Provide a durable conduit for hydrocarbons to flow to the well with acceptable pressure losses (sufficient conductivity) There are certainly more than five deficiencies in our stimulation designs and our techniques to analyze well production. However, the five issues described in this paper include:

	- **2.** Fracture conductivity is not constant. Lab data suggest that all conventional proppant types suffer continued crush and compaction over time.
	- **4.** Fractures develop varying degrees of complexity. This is both good and bad. Reservoir contact is increased as fractures branch, twist, and energize pre-existing planes of weakness. However, this complexity challenges our ability to place a durable, hydrauli‐ cally continuous proppant pack with conductivity commensurate to carry hydrocarbons with an acceptably small pressure loss.
	- **5.** History-matching of production data is surprisingly non-unique. An engineer can reinforce misconceptions throughout an entire career without encountering any results that cannot be matched with a simple, planar frac of durable, high conductivity in a homogenous reservoir. Underperformance can always be attributed to other factors.

While this is a fairly depressing view of the problem, there are techniques to remove some of the uncertainty and ambiguity allowing significant improvement in the performance of stimulation treatments.

#### From a single decline curve, we cannot uniquely determine whether the fracture is short and "infinitely conductive," or long with **8. Removing the uncertainty**

We cannot prove whether the fracture conductivity was constant or degrading. Most engineers attempt to match the data with an analytic solution or a numerical simulator that presumes the frac is fully packed with proppant throughout, providing uniform and Several datasets and techniques can be used to more uniquely describe the performance of propped fractures [19]:

durable flow capacity without collapse of poorly propped sections. Note that with this approach an engineer can continue to reinforce any existing misconceptions. Fracs can be interpreted to be long or short. Disappointing well productivity can always be **•** Wells that are restimulated. When we refrac a well, we have an opportunity to history-match the production from the initial and subsequent stimulation treatments using only a single reservoir description. Difference in well production must be uniquely attributed to the frac design. There have been more than 140 published examples, and history-matching attempts have frequently indicated that fractures are not as effective or durable as previously anticipated [10, 20].

continue experimenting and studying production from wells, with a healthy skepticism of model predictions and of historic rules of thumb regarding fracture design. Another common finding is that emphasis on improving the effectiveness and durability of treatments appears to be adding more value than blindly focusing on fracture length or treatment volume. There are a great number of field examples in which modest changes to fracturing designs resulted in very large changes to well productivity, convincingly demonstrating that our initial frac designs were insufficient to capture the full well potential. Figure 10 shows surprising increases in productivity were achieved by restimulating a modest perm oil reservoir and a tight gas reservoir with improved fracture designs more focused on the durability and

conductivity of the fracturing treatments.

**May-84 May-86 May-88 May-90 May-92 May-94 May-96 May-98 May-00 Date**

Address all correspondence to: mike@fracwell.com

Fracwell Llc, Golden, Colorado, USA

**Incremental Oil Exceeds 1,000,000 barrels**

> **Second Refrac**

**Incremental Oil exceeds 650,000 barrels**

**First Refrac** **Original Fracture (20/40 Sand) Phase I refrac (20/40 Sand) Phase III refrac (16/20 LWC)**

**0**

**Pre Frac 10,000 gal 3% acid + 10,000 lb glass beads**

Five Things You Didn't Want to Know about Hydraulic Fractures

http://dx.doi.org/10.5772/56066

91

**80,000 gal + 100,000 lb 20/40 sand**

**75,000 gal + 120,000 lb 20/40 ISP**

**500**

**1000**

**1500**

**Stabilized Rate (MSCFD)**

**Figure 10.** Experimentation with frac design often demonstrates the well potential is constrained by insufficient frac‐

Similar production increases have been documented in hundreds of field studies in shales, carbonates, coals, and sandstones [1]. On one hand, it is frustrating to admit that after decades we have failed to optimize our fracturing treatments. On the other hand, it is great news that our fracs are not optimized. Reservoirs are often capable of tremendous increases in produc‐ tivity with improved fracture designs that accommodate and capitalize on our understanding

**2000**

**2500**

ture designs [1, 20]

of complexity.

**Author details**

Vincent M. C.\*

**Production from Fracture (bfpd)**


These efforts strongly indicate that additional focus on the conductivity, durability and *effectiveness* of the fracture is needed – not just a focus on created dimensions.
