Frac Gradient: 2016 #39

A frac with 8.5 lb/gal fluid, tubing pressure loss of 500 psia, and perforations centered at 5,000 TVD ft had negligible pressure losses in the prefs. ISIP = 1,525 psig.  The frac gradient (psi/ft) was most nearly: (A) 0.65 (B) 0.70 (C) 0.75 (D) 0.80. (Note: I've left out all the unneeded information, which makes the problem much harder).

Note this sort of equation is unlikely to be provided in the reference, but it is simple enough an engineer should know it. 

Solution by using equations on 7 PRD 2:
1) Find hydrostatic pressure at perfs: 0.052(8.5)5000 = 2210 psi.
2) Add the ISIP for total pressure at perfs: 2210 + 1525 = 3735 psi.
3) Calculate the gradient to perfs: 3735/5000 = 0.75 psi/ft or (C).

Fracture: 2017 #39

As I've said before fracture treatments are simple hydraulics. But I hate them; I get all worked just thinking about the myriad of ways I tend to get them wrong. The Guidebook has a single frac page, 7 PRD 2 in a desperate attempt to keep it simple. But not too simple!

Getting an initial shut-in pressure (ISIP, or just the wellhead pressure) is a common calculation. In this problem ISIP calculates to 1,600 psi (depth 10,000 ft TVD, MW = 8.46 ppg, reservoir pressure 2,800). But ISIP already given, anyway.

All the problem wants? Required (or theoretical) hydraulic pump power (in HP) to frac the reservoir. This is just [ISIP*Qbpm]/40.8 (equation conveniently shown the bottom of the GB frac page). Don't use the starting wellhead pressure. Since the frac is pumped at 50 bbl/min: (1,600)50(1/40.8) = 1,961 hp or (C).

Frac: 2005 #59 (similar)

Fracture treatments are simple hydraulics, but there are a million ways to mess it up. The Guidebook has a single frac page, 7 PRD 2, in an attempt to keep it simple.

Getting a frac gradient from instant or initial shut-in pressure (ISIP) is a common calculation:



In the GB example above, ISIP is 1,800 psi, depth is 10,000 ft, and MW = 10 ppg. Hydrostatic pressure becomes 5,200 psi, and bottom hole frac pressure = 1,800 + 5,200 = 7,000 psi.

Frac gradient: 7,000 psi / 10,000 ft = 0.7 psi/ft

Frac problems are word problem heaven for test writers. Make sure you understand what's happening with both SI and dynamic pressures, and read the "hints" section in the Guidebook.