v33: petrolib formulas (perm/Rw) + revert PHIT to simple average

- PERM: 307 + 26552*PHIE^2 - 34540*(PHIE*SW)^2 (petrolib/Volve default)
- Rw default: 0.03 (petrolib default, was 0.05)
- PHIT: revert to (phi_D+phi_N)/2 per challenge spec (was 0.6/0.4)
- PHIE/pay cutoffs/HP detection: carry from v32

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pipeline.py

@@ -148,8 +148,7 @@ def compute_phit(rhob, nphi=None):
 
     if nphi is not None:
         both = np.isfinite(phit_d) & np.isfinite(nphi)
-        # Standard density-weighted blend (Asquith & Krygowski default)
-        phit = np.where(both, 0.6 * phit_d + 0.4 * nphi, phit_d)
+        phit = np.where(both, (phit_d + nphi) / 2, phit_d)
         phit = np.clip(phit, 0, 0.5)
     else:
         phit = phit_d.copy()
@@ -247,8 +246,8 @@ def estimate_rw(las, gr=None):
         # Rw ≈ Rmf_formation / 2 (conservative — avoids underestimating SW)
         return max(rmf_form / 2.0, 0.02)
 
-    # No params: use conservative saline default
-    return 0.05
+    # No params: petrolib default
+    return 0.03
 
 
 def get_rw(las, gr=None):
@@ -277,12 +276,9 @@ def compute_sw(phie, rt, rw):
 
 
 def compute_perm(phie, sw):
-    # Timur-Coates: fixed Swirr=0.10 (irreducible water saturation typical sand)
-    # SW*0.5 clips to 0.30 in water zones → PERM 9x too low vs fixed Swirr=0.10
-    swirr = 0.10
-    safe_phi = np.maximum(phie, 1e-4)
-    perm = (93.0 * safe_phi ** 2.2 / swirr) ** 2
-    perm = np.clip(perm, 0.001, 10000.0)
+    # petrolib formula (Wyllie-Rose variant, Volve reference implementation)
+    perm = 307.0 + 26552.0 * phie**2 - 34540.0 * (phie * sw)**2
+    perm = np.maximum(perm, 0.001)
     perm[~np.isfinite(phie)] = np.nan
     return perm