From b32e2f6b3224131bc8335db28647a0843f64abdf Mon Sep 17 00:00:00 2001
From: Tor Harald Sandve <tosa@norceresearch.no>
Date: Mon, 25 Nov 2024 15:43:59 +0100
Subject: [PATCH] Guard against small rates in fraction calculation

---
 opm/simulators/wells/BlackoilWellModelGeneric.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/opm/simulators/wells/BlackoilWellModelGeneric.cpp b/opm/simulators/wells/BlackoilWellModelGeneric.cpp
index 4ad94f447..d77c36352 100644
--- a/opm/simulators/wells/BlackoilWellModelGeneric.cpp
+++ b/opm/simulators/wells/BlackoilWellModelGeneric.cpp
@@ -1222,7 +1222,9 @@ updateAndCommunicateGroupData(const int reportStepIdx,
                                                     phaseIdx,
                                                     /*isInjector*/ false);
                         }
-                        Scalar rel_change = std::abs( (gr_rate_nupcol - gr_rate) / (0.5*gr_rate_nupcol + 0.5*gr_rate));
+                        Scalar small_rate = 1e-12; // m3/s
+                        Scalar denominator = (0.5*gr_rate_nupcol + 0.5*gr_rate);
+                        Scalar rel_change = denominator > small_rate ? std::abs( (gr_rate_nupcol - gr_rate) / denominator) : 0.0;
                         if ( rel_change > tol_nupcol) {
                             this->updateNupcolWGState();
                             const std::string control_str = is_vrep? "VREP" : "REIN";