diff --git a/msim/include/opm/msim/msim.hpp b/msim/include/opm/msim/msim.hpp
index 34ce4d342..e22783730 100644
--- a/msim/include/opm/msim/msim.hpp
+++ b/msim/include/opm/msim/msim.hpp
@@ -27,6 +27,7 @@ class Parser;
 class Python;
 class SummaryState;
 class UDQState;
+class WellTestState;
 
 namespace Action {
 class State;
@@ -48,10 +49,10 @@ public:
     void post_step(Schedule& schedule, Action::State& action_state, SummaryState& st, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, const time_point& sim_time);
 private:
 
-    void run_step(const Schedule& schedule, Action::State& action_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, EclipseIO& io) const;
-    void run_step(const Schedule& schedule, Action::State& action_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double dt, EclipseIO& io) const;
-    void output(Action::State& action_state, SummaryState& st, const UDQState& udq_state, size_t report_step, bool substep, double seconds_elapsed, const data::Solution& sol, const data::Wells& well_data, const data::GroupAndNetworkValues& group_data, EclipseIO& io) const;
-    void simulate(const Schedule& schedule, const SummaryState& st, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double seconds_elapsed, double time_step) const;
+    void run_step(const Schedule& schedule, Action::State& action_state, WellTestState& wtest_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, EclipseIO& io) const;
+    void run_step(const Schedule& schedule, Action::State& action_state, WellTestState& wtest_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double dt, EclipseIO& io) const;
+    void output(Action::State& action_state, WellTestState& wtest_state, SummaryState& st, const UDQState& udq_state, size_t report_step, bool substep, double seconds_elapsed, const data::Solution& sol, const data::Wells& well_data, const data::GroupAndNetworkValues& group_data, EclipseIO& io) const;
+    void simulate(const Schedule& schedule, WellTestState& wtest_state, const SummaryState& st, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double seconds_elapsed, double time_step) const;
 
     EclipseState state;
     std::map<std::string, std::map<data::Rates::opt, std::function<well_rate_function>>> well_rates;
diff --git a/msim/src/msim.cpp b/msim/src/msim.cpp
index e34d773d6..f11ec1e0b 100644
--- a/msim/src/msim.cpp
+++ b/msim/src/msim.cpp
@@ -34,6 +34,7 @@
 #include <opm/parser/eclipse/EclipseState/Schedule/Action/State.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQConfig.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/WellTestState.hpp>
 #include <opm/parser/eclipse/Parser/Parser.hpp>
 #include <opm/parser/eclipse/Parser/ParseContext.hpp>
 #include <opm/parser/eclipse/Parser/ErrorGuard.hpp>
@@ -50,6 +51,7 @@ void msim::run(Schedule& schedule, EclipseIO& io, bool report_only) {
     data::Solution sol;
     SummaryState st(TimeService::from_time_t(schedule.getStartTime()));
     UDQState udq_state(schedule.getUDQConfig(0).params().undefinedValue());
+    WellTestState wtest_state;
     Action::State action_state;
     Python python;
 
@@ -58,10 +60,10 @@ void msim::run(Schedule& schedule, EclipseIO& io, bool report_only) {
         data::Wells well_data;
         data::GroupAndNetworkValues group_nwrk_data;
         if (report_only)
-            run_step(schedule, action_state, st, udq_state, sol, well_data, group_nwrk_data, report_step, io);
+            run_step(schedule, action_state, wtest_state, st, udq_state, sol, well_data, group_nwrk_data, report_step, io);
         else {
             double time_step = std::min(week, 0.5*schedule.stepLength(report_step - 1));
-            run_step(schedule, action_state, st, udq_state, sol, well_data, group_nwrk_data, report_step, time_step, io);
+            run_step(schedule, action_state, wtest_state, st, udq_state, sol, well_data, group_nwrk_data, report_step, time_step, io);
         }
         auto sim_time = TimeService::from_time_t( schedule.simTime(report_step) );
         post_step(schedule, action_state, st, sol, well_data, group_nwrk_data, report_step, sim_time);
@@ -95,12 +97,12 @@ void msim::post_step(Schedule& schedule, Action::State& action_state, SummarySta
 
 
 
-void msim::run_step(const Schedule& schedule, Action::State& action_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& grp_nwrk_data, size_t report_step, EclipseIO& io) const {
-    this->run_step(schedule, action_state, st, udq_state, sol, well_data, grp_nwrk_data, report_step, schedule.stepLength(report_step - 1), io);
+void msim::run_step(const Schedule& schedule, Action::State& action_state, WellTestState& wtest_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& grp_nwrk_data, size_t report_step, EclipseIO& io) const {
+    this->run_step(schedule, action_state, wtest_state, st, udq_state, sol, well_data, grp_nwrk_data, report_step, schedule.stepLength(report_step - 1), io);
 }
 
 
-void msim::run_step(const Schedule& schedule, Action::State& action_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double dt, EclipseIO& io) const {
+void msim::run_step(const Schedule& schedule, Action::State& action_state, WellTestState& wtest_state, SummaryState& st, UDQState& udq_state, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& group_nwrk_data, size_t report_step, double dt, EclipseIO& io) const {
     double start_time = schedule.seconds(report_step - 1);
     double end_time = schedule.seconds(report_step);
     double seconds_elapsed = start_time;
@@ -110,7 +112,7 @@ void msim::run_step(const Schedule& schedule, Action::State& action_state, Summa
         if ((seconds_elapsed + time_step) > end_time)
             time_step = end_time - seconds_elapsed;
 
-        this->simulate(schedule, st, sol, well_data, group_nwrk_data, report_step, seconds_elapsed, time_step);
+        this->simulate(schedule, wtest_state, st, sol, well_data, group_nwrk_data, report_step, seconds_elapsed, time_step);
 
         seconds_elapsed += time_step;
 
@@ -127,6 +129,7 @@ void msim::run_step(const Schedule& schedule, Action::State& action_state, Summa
         schedule.getUDQConfig( report_step ).eval(report_step, schedule.wellMatcher(report_step), st, udq_state);
 
         this->output(action_state,
+                     wtest_state,
                      st,
                      udq_state,
                      report_step,
@@ -141,9 +144,10 @@ void msim::run_step(const Schedule& schedule, Action::State& action_state, Summa
 
 
 
-void msim::output(Action::State& action_state, SummaryState& st, const UDQState& udq_state, size_t report_step, bool substep, double seconds_elapsed, const data::Solution& sol, const data::Wells& well_data, const data::GroupAndNetworkValues& group_nwrk_data, EclipseIO& io) const {
+void msim::output(Action::State& action_state, WellTestState& wtest_state, SummaryState& st, const UDQState& udq_state, size_t report_step, bool substep, double seconds_elapsed, const data::Solution& sol, const data::Wells& well_data, const data::GroupAndNetworkValues& group_nwrk_data, EclipseIO& io) const {
     RestartValue value(sol, well_data, group_nwrk_data, {});
     io.writeTimeStep(action_state,
+                     wtest_state,
                      st,
                      udq_state,
                      report_step,
@@ -153,7 +157,7 @@ void msim::output(Action::State& action_state, SummaryState& st, const UDQState&
 }
 
 
-void msim::simulate(const Schedule& schedule, const SummaryState& st, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& /* group_nwrk_data */, size_t report_step, double seconds_elapsed, double time_step) const {
+void msim::simulate(const Schedule& schedule, WellTestState& wtest_state, const SummaryState& st, data::Solution& sol, data::Wells& well_data, data::GroupAndNetworkValues& /* group_nwrk_data */, size_t report_step, double seconds_elapsed, double time_step) const {
     for (const auto& sol_pair : this->solutions) {
         auto func = sol_pair.second;
         func(this->state, schedule, sol, report_step, seconds_elapsed + time_step);
diff --git a/opm/output/eclipse/EclipseIO.hpp b/opm/output/eclipse/EclipseIO.hpp
index bdcb86adc..5e9c1deee 100644
--- a/opm/output/eclipse/EclipseIO.hpp
+++ b/opm/output/eclipse/EclipseIO.hpp
@@ -46,6 +46,7 @@ class Schedule;
 class SummaryConfig;
 class SummaryState;
 class UDQState;
+class WellTestState;
 namespace Action { class State; }
 /*!
  * \brief A class to write the reservoir state and the well state of a
@@ -179,6 +180,7 @@ public:
      */
 
     void writeTimeStep( const Action::State& action_state,
+                        const WellTestState& wtest_state,
                         const SummaryState& st,
                         const UDQState& udq_state,
                         int report_step,
diff --git a/opm/output/eclipse/RestartIO.hpp b/opm/output/eclipse/RestartIO.hpp
index 4ed7703f3..d6d16c640 100644
--- a/opm/output/eclipse/RestartIO.hpp
+++ b/opm/output/eclipse/RestartIO.hpp
@@ -40,6 +40,7 @@ namespace Opm {
     class Schedule;
     class UDQState;
     class SummaryState;
+    class WellTestState;
 
 } // namespace Opm
 
@@ -86,6 +87,7 @@ namespace Opm { namespace RestartIO {
               const EclipseGrid&                            grid,
               const Schedule&                               schedule,
               const Action::State&                          action_state,
+              const WellTestState&                          wtest_state,
               const SummaryState&                           sumState,
               const UDQState&                               udqState,
               std::optional<Helpers::AggregateAquiferData>& aquiferData,
diff --git a/src/opm/output/eclipse/EclipseIO.cpp b/src/opm/output/eclipse/EclipseIO.cpp
index e5e939c06..13c4cc8dd 100644
--- a/src/opm/output/eclipse/EclipseIO.cpp
+++ b/src/opm/output/eclipse/EclipseIO.cpp
@@ -268,6 +268,7 @@ void EclipseIO::writeInitial( data::Solution simProps, std::map<std::string, std
 
 // implementation of the writeTimeStep method
 void EclipseIO::writeTimeStep(const Action::State& action_state,
+                              const WellTestState& wtest_state,
                               const SummaryState& st,
                               const UDQState& udq_state,
                               int report_step,
@@ -318,7 +319,7 @@ void EclipseIO::writeTimeStep(const Action::State& action_state,
         };
 
         RestartIO::save(rstFile, report_step, secs_elapsed, value,
-                        es, grid, schedule, action_state, st,
+                        es, grid, schedule, action_state, wtest_state, st,
                         udq_state, this->impl->aquiferData, write_double);
     }
 
diff --git a/src/opm/output/eclipse/RestartIO.cpp b/src/opm/output/eclipse/RestartIO.cpp
index a9647d7a9..95cd4818d 100644
--- a/src/opm/output/eclipse/RestartIO.cpp
+++ b/src/opm/output/eclipse/RestartIO.cpp
@@ -480,7 +480,7 @@ namespace {
                           const EclipseGrid&                            grid,
                           const EclipseState&                           es,
                           const Schedule&                               schedule,
-                          const data::Wells&                        wellSol,
+                          const data::Wells&                            wellSol,
                           const Opm::Action::State&                     action_state,
                           const Opm::SummaryState&                      sumState,
                           const std::vector<int>&                       inteHD,
@@ -757,6 +757,7 @@ void save(EclIO::OutputStream::Restart&                 rstFile,
           const EclipseGrid&                            grid,
           const Schedule&                               schedule,
           const Action::State&                          action_state,
+          const WellTestState&                          ,
           const SummaryState&                           sumState,
           const UDQState&                               udqState,
           std::optional<Helpers::AggregateAquiferData>& aquiferData,
diff --git a/tests/test_EclipseIO.cpp b/tests/test_EclipseIO.cpp
index 9af2390ff..1209fc782 100644
--- a/tests/test_EclipseIO.cpp
+++ b/tests/test_EclipseIO.cpp
@@ -37,6 +37,7 @@
 #include <opm/parser/eclipse/Units/UnitSystem.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/Action/State.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/WellTestState.hpp>
 
 #include <opm/io/eclipse/EclFile.hpp>
 #include <opm/io/eclipse/EGrid.hpp>
@@ -325,10 +326,12 @@ BOOST_AUTO_TEST_CASE(EclipseIOIntegration) {
             sol.insert("KRG", measure::identity , std::vector<double>(3*3*3 , i*10), TargetType::RESTART_AUXILIARY);
 
             Action::State action_state;
+            WellTestState wtest_state;
             UDQState udq_state(1);
             RestartValue restart_value(sol, wells, grp_nwrk, {});
             auto first_step = ecl_util_make_date( 10 + i, 11, 2008 );
             eclWriter.writeTimeStep( action_state,
+                                     wtest_state,
                                      st,
                                      udq_state,
                                      i,
diff --git a/tests/test_RFT.cpp b/tests/test_RFT.cpp
index 8e64ba228..0137d7a09 100644
--- a/tests/test_RFT.cpp
+++ b/tests/test_RFT.cpp
@@ -41,6 +41,7 @@
 #include <opm/parser/eclipse/EclipseState/Schedule/Action/State.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/SummaryState.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/WellTestState.hpp>
 
 #include <opm/parser/eclipse/Parser/ParseContext.hpp>
 #include <opm/parser/eclipse/Parser/Parser.hpp>
@@ -280,6 +281,7 @@ BOOST_AUTO_TEST_CASE(test_RFT)
         SummaryState st(TimeService::now());
         Action::State action_state;
         UDQState udq_state(1234);
+        WellTestState wtest_state;
 
         data::Rates r1, r2;
         r1.set( data::Rates::opt::wat, 4.11 );
@@ -322,6 +324,7 @@ BOOST_AUTO_TEST_CASE(test_RFT)
         RestartValue restart_value(std::move(solution), std::move(wells), std::move(group_nwrk), {});
 
         eclipseWriter.writeTimeStep( action_state,
+                                     wtest_state,
                                      st,
                                      udq_state,
                                      2,
@@ -409,6 +412,7 @@ BOOST_AUTO_TEST_CASE(test_RFT2)
         SummaryState st(Opm::TimeService::now());
         Action::State action_state;
         UDQState udq_state(10);
+        WellTestState wtest_state;
 
         const auto  start_time = schedule.posixStartTime();
         for (int counter = 0; counter < 2; counter++) {
@@ -456,6 +460,7 @@ BOOST_AUTO_TEST_CASE(test_RFT2)
                 RestartValue restart_value(std::move(solution), std::move(wells), data::GroupAndNetworkValues(), {});
 
                 eclipseWriter.writeTimeStep( action_state,
+                                             wtest_state,
                                              st,
                                              udq_state,
                                              step,
diff --git a/tests/test_Restart.cpp b/tests/test_Restart.cpp
index 5ca08bb8d..e08e5c2bb 100644
--- a/tests/test_Restart.cpp
+++ b/tests/test_Restart.cpp
@@ -44,6 +44,7 @@
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQConfig.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQEnums.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/WellTestState.hpp>
 
 #include <opm/io/eclipse/OutputStream.hpp>
 #include <opm/io/eclipse/EclIOdata.hpp>
@@ -390,6 +391,7 @@ struct Setup {
 
 
 RestartValue first_sim(const Setup& setup, Action::State& action_state, SummaryState& st, UDQState& udq_state, bool write_double) {
+    WellTestState wtest_state;
     EclipseIO eclWriter( setup.es, setup.grid, setup.schedule, setup.summary_config);
     auto num_cells = setup.grid.getNumActive( );
     int report_step = 1;
@@ -404,6 +406,7 @@ RestartValue first_sim(const Setup& setup, Action::State& action_state, SummaryS
 
     udq.eval(report_step, setup.schedule.wellMatcher(report_step), st, udq_state);
     eclWriter.writeTimeStep( action_state,
+                             wtest_state,
                              st,
                              udq_state,
                              report_step,
@@ -485,6 +488,7 @@ BOOST_AUTO_TEST_CASE(ECL_FORMATTED) {
         auto udqState = UDQState{1};
         auto aquiferData = std::optional<Opm::RestartIO::Helpers::AggregateAquiferData>{std::nullopt};
         Action::State action_state;
+        WellTestState wtest_state;
         {
             RestartValue restart_value(cells, wells, groups, {});
 
@@ -507,6 +511,7 @@ BOOST_AUTO_TEST_CASE(ECL_FORMATTED) {
                                 base_setup.grid,
                                 base_setup.schedule,
                                 action_state,
+                                wtest_state,
                                 sumState,
                                 udqState,
                                 aquiferData,
@@ -538,6 +543,7 @@ BOOST_AUTO_TEST_CASE(ECL_FORMATTED) {
                                 base_setup.grid,
                                 base_setup.schedule,
                                 action_state,
+                                wtest_state,
                                 sumState,
                                 udqState,
                                 aquiferData,
@@ -623,6 +629,7 @@ BOOST_AUTO_TEST_CASE(WriteWrongSOlutionSize) {
         Opm::SummaryState sumState(TimeService::now());
         Opm::Action::State action_state;
         Opm::UDQState udq_state(19);
+        Opm::WellTestState wtest_state;
         auto aquiferData = std::optional<Opm::RestartIO::Helpers::AggregateAquiferData>{std::nullopt};
 
         const auto seqnum = 1;
@@ -638,6 +645,7 @@ BOOST_AUTO_TEST_CASE(WriteWrongSOlutionSize) {
                                            setup.grid ,
                                            setup.schedule,
                                            action_state,
+                                           wtest_state,
                                            sumState,
                                            udq_state,
                                            aquiferData),
@@ -676,6 +684,7 @@ BOOST_AUTO_TEST_CASE(ExtraData_content) {
     Setup setup("BASE_SIM.DATA");
     {
         Action::State action_state;
+        WellTestState wtest_state;
         UDQState udq_state(10);
         auto num_cells = setup.grid.getNumActive( );
         auto cells = mkSolution( num_cells );
@@ -706,6 +715,7 @@ BOOST_AUTO_TEST_CASE(ExtraData_content) {
                                 setup.grid,
                                 setup.schedule,
                                 action_state,
+                                wtest_state,
                                 sumState,
                                 udq_state,
                                 aquiferData);
@@ -786,6 +796,7 @@ BOOST_AUTO_TEST_CASE(STORE_THPRES) {
             const auto sumState = sim_state(base_setup.schedule);
             Action::State action_state;
             UDQState udq_state(99);
+            WellTestState wtest_state;
 
             /* THPRES data has wrong size in extra container. */
             {
@@ -802,6 +813,7 @@ BOOST_AUTO_TEST_CASE(STORE_THPRES) {
                                                    base_setup.grid,
                                                    base_setup.schedule,
                                                    action_state,
+                                                   wtest_state,
                                                    sumState,
                                                    udq_state,
                                                    aquiferData),
@@ -827,6 +839,7 @@ BOOST_AUTO_TEST_CASE(STORE_THPRES) {
                                 base_setup.grid,
                                 base_setup.schedule,
                                 action_state,
+                                wtest_state,
                                 sumState,
                                 udq_state,
                                 aquiferData);
@@ -884,6 +897,7 @@ BOOST_AUTO_TEST_CASE(Restore_Cumulatives)
     const auto seqnum = 1;
     {
         Action::State action_state;
+        WellTestState wtest_state;
         auto aquiferData = std::optional<Opm::RestartIO::Helpers::AggregateAquiferData>{std::nullopt};
         auto rstFile = OS::Restart {
             rset, seqnum, OS::Formatted{ false }, OS::Unified{ true }
@@ -891,7 +905,7 @@ BOOST_AUTO_TEST_CASE(Restore_Cumulatives)
 
         RestartIO::save(rstFile, seqnum, 100, restart_value,
                         setup.es, setup.grid, setup.schedule,
-                        action_state, sumState, udq_state, aquiferData);
+                        action_state, wtest_state, sumState, udq_state, aquiferData);
     }
 
     Action::State action_state;
diff --git a/tests/test_restartwellinfo.cpp b/tests/test_restartwellinfo.cpp
index 06e4d7a0f..8d7005de9 100644
--- a/tests/test_restartwellinfo.cpp
+++ b/tests/test_restartwellinfo.cpp
@@ -40,6 +40,7 @@
 #include <opm/parser/eclipse/EclipseState/Schedule/Well/WellInjectionProperties.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/Action/State.hpp>
 #include <opm/parser/eclipse/EclipseState/Schedule/UDQ/UDQState.hpp>
+#include <opm/parser/eclipse/EclipseState/Schedule/Well/WellTestState.hpp>
 
 #include <opm/io/eclipse/EclFile.hpp>
 
@@ -222,10 +223,12 @@ BOOST_AUTO_TEST_CASE(EclipseWriteRestartWellInfo) {
     solution.insert( "SGAS"    , Opm::UnitSystem::measure::identity , std::vector< double >( num_cells, 1 ) , Opm::data::TargetType::RESTART_SOLUTION);
     Opm::data::Wells wells;
     Opm::data::GroupAndNetworkValues group_nwrk;
+    Opm::WellTestState wtest_state;
 
     for(int timestep = 0; timestep <= countTimeStep; ++timestep) {
 
         eclipseWriter.writeTimeStep( action_state,
+                                     wtest_state,
                                      st,
                                      udq_state,
                                      timestep,