Question 6 of 7: Two Flow Tests — True Skin and Non-Darcy Factor

Question 6: Two Flow Tests — True Skin and Non-Darcy Factor (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Apparent (test) skin $S_1'=+1$ at $q_1=30{,}000$ MSCFD; $S_2'=+3$ at $q_2=50{,}000$ MSCFD.

Find. True (rate-independent) skin factor $S$; non-Darcy coefficient $D$; a recommendation on acidizing.

Approach. The formula sheet’s relation $S'=S+Dq$ is linear in $q$ with two unknowns ($S$, $D$) and two tests — solve the resulting 2×2 linear system directly, no chart needed.

1. Set up the two equations. $S_1'=S+Dq_1\Rightarrow1=S+D(30{,}000)$; $S_2'=S+Dq_2\Rightarrow3=S+D(50{,}000)$.
2. Solve for $D$. Subtracting: $S_2'-S_1'=D(q_2-q_1)\Rightarrow3-1=D(50{,}000-30{,}000)$: $\boxed{D=1.0\times10^{-4}\ \text{MSCFD}^{-1}}$.
3. Solve for $S$. $S=S_1'-Dq_1=1-(1.0\times10^{-4})(30{,}000)$: $\boxed{S=-2.0}$.
4. Acidizing recommendation. The true (rate-independent) skin is $S=-2.0$, i.e. negative — the near-wellbore zone already flows better than an undamaged, non-stimulated completion would (consistent with natural fracturing or a prior stimulation treatment). The entire apparent skin observed at each test rate ($+1$ and $+3$) is the rate-dependent non-Darcy (turbulent) term $Dq$, not formation damage. $\boxed{\text{Do NOT recommend acidizing}}$ — acidizing removes near-wellbore damage, and there is none to remove; it would not reduce the turbulent pressure loss, which is a rate effect, not a damage effect.