BUG: fix issue in swfunction when x value gets zero

Author: jcrivenaes

src/fmu/tools/properties/swfunction.py

@@ -76,7 +76,7 @@ class SwFunction:
 
     grid: xtgeo.Grid
     # all input are constants or xtgeo grid or gridproperty
-    x: float | xtgeo.GridProperty = 0.0
+    x: float | xtgeo.GridProperty = 1e-6  # default to avoid division by zero
     a: float | xtgeo.GridProperty = 1.0
     b: float | xtgeo.GridProperty = -1.0
     ffl: float | xtgeo.GridProperty = 999.0
@@ -163,6 +163,9 @@ def _process_input(self) -> None:
                 new_b,
             )
 
+        # ensure that self.x is not zero, as this will cause division by zero
+        self.x.values = np.ma.where(self.x.values == 0.0, 1e-6, self.x.values)
+
     def _compute_htop_hbot(self) -> None:
         """Setting geometries for 'bottom' and 'top' cell.
 
@@ -321,4 +324,11 @@ def compute(self, compute_method: str = "integrated") -> xtgeo.GridProperty:
         else:
             self._compute_direct()
 
+        # ensure that the result's mask is not inconcistent with the grid
+        actnum = self.grid.get_actnum()
+        grid_mask = np.ma.masked_where(actnum.values == 0, actnum.values)
+        assert np.array_equal(grid_mask.mask, self._sw.values.mask), (
+            "Bug: Grid mask and Sw mask are not equal, contact developer!"
+        )
+
         return self._sw

tests/properties/test_swfunction.py

@@ -42,6 +42,54 @@ def test_swj_simple(avalue, bvalue, ffl, direct, cellmethod, expected_mean):
     assert sw.values.mean() == pytest.approx(expected_mean, rel=0.01)
 
 
+def test_swj_simple_x_zero():
+    """Test that masked cells are properly treated"""
+
+    grid1 = xtgeo.create_box_grid((2, 3, 1), increment=(1, 1, 1), origin=(0, 0, 0))
+    poro = 0.3
+    perm = 300
+
+    xvalues = math.sqrt(perm / poro)
+
+    xprop = xtgeo.GridProperty(grid1, values=xvalues)
+    xprop.values[0, 0, 0] = 0.0  # Set one value to zero
+
+    sw_obj = SwFunction(
+        grid=grid1,
+        a=1,
+        b=-2,
+        x=xprop,
+        ffl=12.5,
+        invert=True,
+        method="cell_center_above_ffl",
+    )
+    sw = sw_obj.compute("integrated")
+    assert not sw.values.mask[0, 0, 0]
+
+    # now make a grid with masked cells
+    grid2 = grid1.copy()
+    act = grid2.get_actnum()
+    act.values[0, 0, 0] = 0
+    grid2.set_actnum(act)
+
+    assert grid2.get_actnum().values[0, 0, 0] == 0
+
+    xprop = xtgeo.GridProperty(grid2, values=xvalues)
+    xprop.values[0, 0, 0] = 1000.0
+
+    sw_obj = SwFunction(
+        grid=grid2,
+        a=1,
+        b=-2,
+        x=xprop,
+        ffl=12.5,
+        invert=True,
+        method="cell_center_above_ffl",
+    )
+    sw = sw_obj.compute("integrated")
+    assert sw.values.mask[0, 0, 0]
+
+
 def test_swj_simple_threshold_2grids():
     """Test a simple SwJ setup, expected mean are checked with RMS Sw job.